The cycle of 4ths is one the most useful tools for practicing, it has other uses as well, but for right now lets just talk about what it is. The Cycle of 4ths is a 12 note cycle that spells out all 12 keys, all of the notes are a perfect fourth apart.
A perfect fourth refers to an interval, now dont freak out, an interval is the distance between two notes. The smallest interval we can have in music is a half step. Music only has 12 half steps. A perfect 4th is 5 half steps. Of course there are other types of intervals, but for now lets just concentrate on 4ths.
Lets start with C and count up 5 half steps, if you have done this correctly you should end up on F. Now go 5 more half steps you should end up on Bb. Eventually if you keep doing this you will eventually start over. It is to your advantage to memorize the cycle of 4ths.