I am 16 years old, have finished my letters on both piano, violin and viola (which has alto clef as well). Here are my tips;
Count out loud as you play, and dont start off too fast.
Practice one short piece each day
if you make a mistake, keep going at the same tempo
Rhythm is more important than notes, so keep the pace and the rhythm.
Dont waste your time worrying about sight reading and if you can, start looking at doing some accompanying, or joining an orchestra or band.
MOST OF ALL though, sight-reading what playing an instrument is about. I've been playing for over ten years and after it comes to you, through hard work, its the best feeling in the world