Your question is very broad. Keep in mind that not everything on your computer is transmitted via wifi.
Monitor my desktop activity?
View documents I've saved on my computer?
Tell what sites i visit?
These aren't exactly functions of having wireless internet. I mean if you save a document to your local hard drive it would never be on the internet (or any network for that matter) so wifi is out of the picture.
Tell what my passwords / logins / email addresses are?
This has more to do with wifi. You want to make sure the channel you use to connect to your wireless access point (e.g. router) is secure so that others in the area can't snoop what's being transmitted.
To find out if your being monitored could be lengthy proccess since there's so may ways to monitor someone. Do you have guesses where to start looking? e.g. on your computer itself or on the router.