Hey-
My phone is running really really slow. I have a lot of applications, is this why? I don't want to delete them! When I say a lot I mean around 15.
2nd Question.
Can you root with a Mac? I know nothing about rooting. Why should I do it?
Thank you
-Kristin
First question-15 apps is not a lot

Don't worry about that. However, if you have a lot of widgets running, that could definitely slow things down, as could improper use of a Task Killer if you are running one.
Second question-I used my Mac to root, but I did it in Boot Camp. do recall several threads about this that you can probably find with a search (HINT: Use the Google Search rather than the native search and you will have much better results).
Now, that is assuming that you are going to use the RSD method (which I would recommend). There are several apps out there, such as Easy Root, that say they will root for you with the press of a button. There have been a lot of threads about people having problems with this app. That could be due to a problem with the app or root process, or simply due to the fact that this encourages uninformed and unprepared rooting. We are all here to help, but PLEASE research and study the subject before diving in.
Check out the Droid Hacking section for some really good resources.