When I hold my mouse still, it often moves upward slowly & often to the left. It's not smooth movement either, it jiggles while it's moving. How do I stop this?

Sounds like a similar problem I had with an optical mouse.
I fixed it by updating the drivers and making sure it was plugged into a USB port rather the a PS/2 port that it was.

Also make sure that the mouse is not on a reflective surface (glass, highly polished table, etc.), try putting the mouse on a dark matte surface and see what happens.

If you use an optical mouse, be sure the sender is not blocked by something, it needs to see the mouse.

When you say "sender", are you referring to a wireless mouse. If not, where is the sender?

when you say "sender", are you referring to a wireless mouse. If not, where is the sender?Yes, he means a "wireless" mouse. The sender he refers to is most likely the base station or receiver.