When famous professional musicians are interviewed, they are often asked, "What advice can you give players who are just starting out?" Here's a list of their most common answers.
1) Practice on a good-quality and well-set-up guitar.Why make learning the guitar more difficult than it needs to be? Why struggle on a guitar that plays out of tune and has a high action (i.e. strings far from the fret board)? A pro player would find it difficult to play good music on a bad instrument, so give yourself a good chance of learning to play: Buy a decent instrument and get a professional set-up from a guitar store or luthier, or me.
2) Have a clear goal. For example, sing and play a Beatles song; play a blues solo on electric guitar; play a song at an open mic night; video yourself playing and put it on youtube so you can impress your mum; play at a local jam session; form a rock band with friends; play five acoustic guitar songs well for your own pleasure.... People who have a clear goal do much better than people who vaguely say "I want to learn guitar".
3) Practice often. Even a little practice every day is better than one long practice session at the end of the week just before a guitar lesson. Habits are better formed and reinforced steadily over a period of time.
4) Practice perfectly. Each time you practice try to make it perfect, without mistakes. When first learning a piece of music, the way to do this is to focus on small sections (sometimes just a few notes) and play very slowly. Pay attention to your timing and the quality of each note or chord. Skill is built by playing small sections slowly and accurately, and then repeating this. Soon enough, you will be able to practice longer sections (ie several small sections together). You don't need to force yourself to go faster: keep practicing slowly and the speed will come as your fluency increases.
Poor players often practice by trying to get through a complete song at full speed, before they are ready to do this. They make lots of mistakes and then repeat their mistakes again and again until they become habit.
5) Steadily build on the techniques and skills you have. Choose next songs carefully. If you choose a complicated, advanced or long piece of music before you are ready for it, you could quickly become demotivated due to lack of progress, and then bored with practicing it.
6) Use a metronome. Perhaps not when you are first learning a piece of music, but as soon as you begin to play a section of a song as a whole. The metronome helps you to keep in time - something that is very difficult to judge yourself when you are concentrating on where to put your fingers, coordinating you two hands, remembering what comes next, etc.... The metronome also helps develop the skill of dividing your attention between listening to yourself and another sound. This is good training for playing with other musicians.
7) Record yourself and listen back. This is for a similar reason as using a metronome - it's difficult to hear all aspects of your playing when you are concentrating on playing. Get used to listening to yourself, it's a valuable feedback tool and lets you know you which parts of your playing need more attention (e.g. timekeeping, groove, quality of chord playing or single notes, etc...).