how to create a power up?

I have a tank game for a computer applications class. the tank can shoot enemies and destroy them and get points in the fastest amount of time possible. How do i make power ups?. I already have the energy capsules which are the power ups, but i want to know how to make it so if the tank touches or rolls over a energy capsule it will randomly do one of the following so that the same capsule doesn't do the same thing every time:

make tank bigger
make tank smaller
add point to score
and destroy the tank

[jediaction]

Make it when the tank goes over the power up thing, it activates a method which turns on variables that can be used for different things like increased power or different controls. Just think it out. Make variables for each power up then make them activate once you go over the power up. Make them activate in methods