Sound on Sound magazine has run a number of excellent articles on exactly this subject (try
for starters) but be warned, there is a lot of reading there.
So - given that it's a simple question, how do we come to have so many words written to answer it? Suggestions range from the enigmatic "there are no rules" to the pragmatic "put a SM57 1" from the grill cloth 3" from the centre of the cone at 12 degrees", to the mind numbing issues of phase alignment of multi mic set-ups. The problem is, when you are starting out you have so many variables that it's easier to get lost than to get somewhere. You can change (just a selection here) guitar, guitar controls, pickup, amp, amp position in the room, amp gain, amp volume, amp tone controls, pedals, cabs and speakers, number of microphones, mic distances from the speaker, mic positions on the speaker, mic angles on the speaker, preamp, preamp settings........... ! Add to this that you will most certainly read that every microscopic adjustment of anything will make the difference of "night and day" to your recordings, and that unless you get EVERYTHING right you will suck and it can a) be a bit daunting and b) explain why this question is so often asked and the asked again on a different forum.
Is there an answer? Of course there is, there a lots of them! But let's borrow from the late and great Douglas Adams and start with "DON'T PANIC". OK, that's most of the problem solved. Here's the rest of it. This stuff can be very complex, but it can also be very simple so let's play to our strengths and KISS for the moment. Firstly, discard all ideas of multi-micing. It can sound fantastic and offers a lot of useful options but if you are struggling right now to get one mic to sound good you don't need the hassle. That just got-rid of about 3,000 decisions that you don't need to make. Feeling better yet? Good - so what mic to use? If you have bought lots of mics and don't know what to use them for pick one and put the others away for now. Which one? The world is not going to end if you pick the "wrong" mic, it really doesn't matter that much if you choose the 3rd best mic for the job rather than the 1st best. Dynamic mics tend to be less sensitive so they are harder to overload, they are less sensitive to room sound, they don't (usually) have switches and other controls to worry about, they mount easily on a stand without needing a shock-mount and they don't require phantom power, so if you have a dynamic mic like a SM57, or an Audix I5 then use that (but it really doesn't make THAT MUCH difference).
So far we have a single (probably) dynamic mic to work with. For the record, this is Andy Sneap's usual preference and he manages to do this stuff to a pretty "pro" level! (If you don't know who Andy is go Google).
What next? Choose a guitar that works, plug it into an amp that works and play. Set the controls on the guitar and amp to give you a sound that you like - or just set them to around the middle of their ranges and then leave them alone. This is going to be a couple of minutes' work - don't obsess. If you have a looper pedal or something like a Line 6 Back Track use it now and record 10 or 20 seconds of something direct from the guitar - some single notes, chords, chugs; just a bit of each. Using the looper means that you don't have to do the next bit with a guitar in your hands, and you have an identically played sample each time. If you don't have such a gadget consider buying one or perhaps borrow a tame guitarist.
Now take a sheet of paper and write-out the following list.
1) Centre of cone touching grill cloth
2) Centre of cone 1" from grill cloth
3) Centre of cone 6" from grill cloth
...then write the same options for "edge of dustcap" (this is usually a couple of inches to the side of the centre of the cone) and again for the edge of the speaker. Note that we are talking here about the speaker - not the cabinet.
Set-up your DAW, plug in your mic, set a level on the amp that won't get you arrested, divorced or made homeless. Start your loop running and listen through headphones to the sound that the mic is picking-up. Make sure you mute your monitors and use the most isolating headphones (you really do need to use closed-back 'phones for this) you have (I'm presuming that you don't have the luxury of an isolated control room here). You will get bleed which means that you will hear the amp in the room as well as the sound from the mic but we can't help that. Try putting a hat or headband over the 'phones to make them fit snugly if you need to (so long as no-one can see you, obviously - we're supposed to be cool here). I sometimes use earplugs (I have some with filters that maintain a fairly decent overall balance - most kill the top-end dead) and crank the cans loud to get some additional isolation but be careful, none of this works if you destroy your hearing. Listen for a couple of minutes as you move the mic about in front of the speaker and get an idea of the changes that it makes. Then record all of the options that you have listed - all 9 of them. Write the take number against the list so that you can listen back later - you WILL get confused about which is which. Then listen back. Watch your levels - the centre close sounds will be louder than the edge of speaker sounds and this can wreck your perception - adjust as necessary in your DAW (you could normalise the files?!?) . Be warned, at this point you will most likely hate everything you have recorded. Set-up an EQ that allows you to high-pass at around 80 - 100Hz and low-pass at around 8 -10 KHz. See how much of a difference this makes to reducing the flump and easing the horrible fizz that you probably got. Play around for a few minutes and listen to what happens but again, do not obsess. Chose the 1 or 2 positions that you like best (or hate least) and try again with 1/2 as much gain (up the volume to compensate) and twice as much gain (again, adjust the volume to compensate) and listen to the result. If you have a clear issue with high or low end, adjust the amp and try again or perhape take off all the bass, all the top, all the mid - play around and listen.
Ignore all other options - you can come back to this later. Somewhere in the samples you have recorded is a useable tone. You now need to try this in context, so record some guitar into a song, you know, with drums and bass and possibly a singer and keys. What sounds thin and edgy on it's own plays quite differently when it's fighting for space in a mix.
At this point you will have a good idea of the effect of the major changes that you can make and you have a chance to flex your plug-in or processor muscles. Smash your recording with compression, parallel compress, try some radical EQ, anything you like - and listen to what happens. Don't hurry and don't be afraid to play - this stuff takes time. Somewhere in the options you have tried is a tone that is plenty good enough - perhaps not perfect and probably not your ideal tone, but it will do the job. You now have the basics - congratulations, you can record guitar.
With all of this firmly in place and your ears tuned to the job in hand you can then try all the perfectionist stuff, angling the mic, adding another mic (read-up on phase issues first), different mic positions, different mics, amps, guitars, picks, pickups, different brands of batteries in your pedals, does George L cable sound different to Monster cable, what if I record on a Wednesday................................ . Note that YMMV and you might just become an obsessive freak - welcome to the mad-house!
A couple of pointers for the next step - many mics have a proximity effect that means that the bass response is boosted when they are close to a source - if your bottom is loose and flumpy try moving the mic a little further away. Most mics also have a reduced top-end response off axis - when they are not pointing directly at the source, so if everythign is too bright try angling the mic a few degrees.