You should make a (dynamic) array containing information about all the bullets. Something like this:

[pascal]
TBullet = record
Used: Boolean; //Whether the bullet is in use (visible and moving)
X,Y: Integer; //Position of the bullet
DX,DY: Integer; //Direction in which the bullet travels (you can leave this out when you want all bullets to travel upwards).
//etc etc...
end;

MyBullets: array of TBullet;
[/pascal]

Once in a while, you should loop through this array and move all the bullets one step. After you have done this (or before you make the step), you should check if the bullet has hit something.

Good luck