Can someone tell me how to find what kind of ram I will need for an old dell dimension 2400?
Probably DDR memory. Either PC2100 or PC2700. You'll need to worry about the voltage on the new ram. The new ram should operate at the same voltage the old ram did. If you take out the ram and look at it, it should have the speed and maybe voltage marked on the sticker.
Get PC3200 if you're not sure, just make sure the voltage is the same as the old ram.
What operating system are you using, and how many memory slots do you have?