Chords
The first thing to remember when you are practicing guitar is that there are numerous ways in which you can play chords. If your fingers aren't quite long enough to fret the chord in one way, all you have to do is find a chord further up the fret board. You can also use a clamp-on capo to bring the chords further up the fretboard where the frets are positioned closer together. This not only allows you to more easily finger the chords, but it can compensate for youths using full-size guitars for which their arms may not be long enough.
Strumming the Strings
There are three different ways of plucking the strings. You can finger pick them, which uses all of the fingers of your dominant hand plucking each string in turn, or you can strum the strings, which is how you sound chords. The third way utilizes either a pick or your thumb, strumming one note on one string at a time. This is typically the style of picking used for playing leads or solos.
Reading the Music
If you are unfamiliar with reading music, make a concerted effort to learn. While tablature is a widely used form of notation in guitar playing among amateurs, it is not taken seriously among professional guitar players. It is particularly important to learn to read standard notation if one of your goals is to play classical guitar in an orchestral setting. Many classical pieces are not notated in tablature, and if you can't read music, you will likely not be invited to play with the orchestra, regardless of your skill level.
Repetition
Repetition is the single most important aspect of practicing a piece of music, particularly if that piece is to be played on stage. This is where an electric guitar comes in handy. It produces just enough sound unamplified for you to hear what you're doing, but is quiet enough to ensure that your neighbors or your parents won't go crazy hearing the same song over and over again. After you've repeated the song and know it as well as you think you need to, step away from the music and let your mind wander while you play. You'll find that your practice time will greatly improve your playing if you've repeated the song enough times that you can play it practically without thinking about it.