I know what one of them is なくてはいけません and the other one なければなりません, I just don't know which is which, also a few example sentences would help a lot, thank you.
Obviously ・・・しなくちゃ is a contracted form of ・・・しなくては（なりません）/・・・しなくては（いけません）.
・・・しなければ can be contracted into ・・・しなけりゃ but we don't say '・・・しなけりゃ。' to mean 'I gotta do...'
And I think ・・・しなきゃ is a contracted form of ・・・しなけりゃ（いけない）。
You can also say 勉強しないと。/勉強せねば。