Ok so ive had my rat about 2 and a half years and i want to get a new rat. Should i get a rat the same age as mine or younger, oh ya she's a girl by the way. And how do i make shure the new rat dosent give mine a desise.

Generally, most rats will get along fine as long as they have been properly introduced. There is no way to guarantee they will before you try introductions, but that doesn't mean you shouldn't try; they're very social and should be kept in same sex pairs/groups.

Any time you get a new rat, it needs to be quarantined in another airspace (meaning another house) for three weeks so you can watch it for signs of illness and parasites. After that, you'll have to do proper introductions so they can live together.
How to do introductions: http://ratfanclub.org/newrat.html & http://fatratcentral.com/interacting-wit