Practice

If you're going to learn, you need to put in some practice time.  If you want to become great, you'll need to put in a lot.  Learning exercises, studying theory, taking lessons can all help you become a better player, but even then you'll only get something out of them by sitting down and practicing what you've learnt.

Practice Regularly

Practicing for 10 minutes every day is much better than practicing for 4 hours every sunday.  We're all creatures of habit, so see if you can make practicing a part of your routine.  Chances are you watch television or use the internet in the evening, and you might find yourself spending time looking at stuff you don't even enjoy that much (I know that I do).. if you find yourself doing that, see if you can't have more fun tooling around on the guitar instead.  Soon enough you'll be playing every day just out of habit :)

Keep it fun

If you're trying to learn things you just don't enjoy, or are tackling pieces that frustrate you, then you will naturally be inclined to avoid playing.  Obviously, if you're trying to learn, this is no good at all!  There's no point in taking on songs or exercises if you're just going to end up avoiding them because it's a chore.  It's much easier to keep practicing and learning if you are having fun.

Challenge Yourself

If you want to get better, you need to keep trying new things.. this might mean new songs, new scales and modes, different ideas, different techniques.  If you just keep playing the same thing all the time then you'll just stay in the same place with your playing.  And once you are feeling bored or stuck in a rut playing the same old thing over and over again, it will stop being fun.

Copy players you like

Sometimes there's nothing better than whacking on a CD of music you enjoy, and jamming along.  Even when the music is far more difficult or technical than you're normally used to, you can often latch onto a few things here and there.  And even the simplest songs have little gems of nuance that you can learn from.  No need just to copy guitarists either.. deciphering basslines can often give you an appreciation for how the guitar part works with the rest of the band, while trumpets and saxaphones can be great sources of ideas for solos and melody playing

Experiment

Play around.  Wondering what it would sound like if you did such-and-such?  Give it a try.  Maybe you'll like it - maybe you won't.  Find out.

Use a metronome

To get great technique and a feel for rhythm, play against a click every now and then.  If you are having trouble with a piece, the best way to learn it is to get a metronome and play along to it - start out slow and then gradually speed up.  No need to practice with it all the time - in fact it's probably best if you don't - but use it now and then.

Don't worry too much about gear

Particularly if you're just starting out.  Guitars and amps are lots of fun, and the best ones can make a great player sound fantastic.  But don't let what you own put you off from practicing.  You don't even need an amp to practice at home.  And a cheap guitar is fine for learning on, particularly if you get it set up by a guitar tech.

Just do it

It's all very well to read articles on the internet, but sooner or later you've got to pick up a guitar and play.  Why not do it now :)