my ex used to do her hair blonde, all she ever told me is you can buy the box stuff but youve got to spend a little money than youd normally like, otherwise it'll make your hair crazy lol...also I don't know if this is the same for dying it brunette but try to stay away from the pool for a couple of days...she went into the pool one time and her hair turned green a little lol it was hilarious for me, embarassing for her...good luck - sorry I can't be more of help, just givin you advice as to what my ex used to do...or just go get it done professionally first and ask them there what's good to use - they should know
I think you'd look really good with brown hair, the only thing is that I would get it professionally done unless you've done it before... I know it's expensive, but it's worth it...
If you still want to do it from home, make sure when you look on the box what color it gives in the end... if it has even a little hint of redish at any point, then you can be sure it will look red... so you basically just need to check.
Good luck and make sure to post a picture of your new do... :)