There are many things that one must learn when playing guitar or learning to play guitar. Two really important tips to remember are to relax and to have fun playing. Relaxed play causes less errors. When you are relaxed, you can have fun. When you are having fun you are relaxed. Therefore those two schools of thought go hand in hand when learning to play guitar.
It's important to remember not to compare yourself to other players. There will always be a player that isn't as skilled as you and there will always be a player that is more skilled than you. While sharing ideas and techniques is a good thing, just remember not to compare your skills with the other player.
Playing a practice piece slower than it's intended to play helps you perfect each step of the piece. By helping you to commit those pieces to memory, playing slowly helps your brain connect with the piece and therefore helps to improve your play when played at the proper speed.
Creating your own exercises to improve where you have weaknesses is a great idea as well. You have to be honest with yourself on where your weaknesses lie however. If you are not honest with yourself on your weaknesses, you won't practice the right areas and will not improve in those areas.
Studying drums, piano or violin is a great way to improve your guitar playing. Learning to play the drums helps you understand the beat that is created by the drums, which in turn improves your ability to play with the beat. Learning the different instruments and how music is set up for those other instruments improves your ability for creative play. There is no reason you can't take a piece of music intended for the piano and play it on a guitar.
Always, always, always remember to warm up before you practice. Get into the habit early on in your studies and remember to maintain it every time you play. Cramps and pain caused by a lack of warm up will interfere with your playing and may end a practice session or worse yet a gig that you've been hired to play.
While more expensive, you should practice and play with quality equipment. Less expensive equipment can result in lower quality sounds or the wrong sounds altogether, making it appear that you are not playing properly. This can result in frustration with the guitar and will cause you to quickly lose interest in pursuing the skill further.
Play along with your favorite songs to improve your play. We all sing along with our favorite songs because we enjoy them. Playing your guitar along with your favorite songs will help you improve your play and help you hear any variances that may exist between the recorded version and your actually playing of the piece.
Record yourself playing and listen to the recording. You will first find that you may sound totally different than you thought you did when you play back the recording. However, by listening to the recording you can figure out your weaknesses and improve them.