How do I install my cartridge?

It may be worth it to employ the services of your local stereo dealer, at least if you don't have a knowledgeable buddy.

Having say that, some basic tools and the instructions go a long way. YouTube features videos on cartridge mounting. You may wish to seek help from fellow member who live in your area.

... and I don't have a screwdriver. Do I need one?

Normally yes.
But it is possible that yours is Sirius compatible, did you try to give an order via iPhone4s: "Mount yourself"?

Glen makes some great points. I definitely recommend getting someone more experienced to show you the ropes.

Another invaluable tool is www.vinylengine.com - they have alignment protractors that you can print out and tons of advice. Just make sure that you pay attention to the scale instructions if you use their protractors. It's a great free resource.

In most cases, properly installing a cartridge requires at least a little knowledge and ability. If you don't have a screwdriver, don't know where to mount it, and can't get it off the glass thingy I think you may be miles away from doing a proper job.
Have someone do it for you.

The most frequent accidents in installing a cartridge is tearing off the clip on the end of the arm wire. This is because people try to put the clip onto the installed cartridge pin with a pair of pliars...it slips, and off comes the clip. I have found that it will not happen if you wire the cartridge BEFORE installing it in the arm. Push the cartridge pins on to the proper colored wires by holding the cartridge in your left hand and pusing the cartridge pins on to the wires and then attaching the cartridge to the headshell. Zei Gazundt.