My roommate keeps a lot of junk food in the kitchen and lately I've been having the urge to ask them not to do that.
Well where else would she keep the junk food at?
I don't know but it being in the kitchen makes me crave it and I'm trying to eat healthier.
It's completely understandable to want your roommate to keep the junk food out of your sight if you're trying to eat healthier. Asking her to do so should be fine and hopefully she's willing to help out. Just make sure not to demand her to do it.
I also want her to start exercising since she doesn't do it right now.
It seems like you want both of you to have a healthier lifestyle, that's fine. I just recommend not being too intrusive when you explain this to her.