First, both Ganbatte and Ganbareba come originally from the verb "がんばる(Ganbaru)", meaning to work hard on something and to break a leg. So the ways to use the verb are different grammatically.
がんばって(Ganbatte)
If you say ”Break a leg!" to someone in Japanese, you say "Ganbatte!". And if you want to be more polite, you can say "Ganbatte kudasai!"
For example
A:明日の面接が心配だな。。。(I'm getting nervous about an interview tomorrow...)
B:Ganbatte!(Break a leg!)
がんばれば(Ganbareba)
"------reba" means "if --------------," in English. It's one of Japanese conjunctions.
For example, 「日本語の勉強をがんばれば、日本人と日本語でおしゃべりできるようになる。(Nihongo no benkyou wo ganbareba, nihonjin to oshaberi dekiru youni naru.)」 (If you work hard on learning Japanese, you'll be able to chat with them in Japanese.)
Ganbatte! ^^