What area of your life do you want to work on? Formulate a clear concise sentence about some goal or some behavior or anything at all that you want in the near future.
Write it down. If you have lots, then write them all down.
Then just choose which tape seems to fit #1 on your list.
If you are a studying & in school, then do Memory Supercharger b/f going to bed at night. Follow the directions on the tape b/c it says to do it daily for a full week. Doing it longer is fine, i know i have used it longer than 7 days. Doing it b/f sleep is also very worthwhile.
good luck 
One thing i've noticed is that i sometimes use the Paraliminals a lot & then i just feel saturated & ok & i stop. Recently i listened to Self Esteem till i just felt good about myself w/ a particular project i was oscillating on. You just have to get a "feel" for them.