Hi,

Im a newbie, going to get my first mic -- be gentle!

Going to purchase sennheiser 416 and a mackie mixer. What is a preamp, and do I need one? If I need one, could you recommend a reliable, cheap one? (I know you get what you pay for!)

Thanks for your time!

What kind of recording work are you trying to do? That mic and mixer is a bit of an odd combination because the 416 is generally for portable outdoor recording and Mackie mixers are almost always used in a fixed location with AC power.
A preamp is necessary to bring a mic-level signal up to line-level.
Cameras with mic inputs have this built in. Mixers have it built into their mic inputs.
A separate preamp is a specialized device just for pre-amplifying a mic signal before sending it to a device that uses line-level signals.

A mixer is a pre-amp with mixing abilities, so no you don't need a pre-amp because you'll already have one built in. Make sure you research your mixers and get one for DV audio, not a DJ-type mixer. They are different.