「行かないといけない」 => 行く　なけれ ば　いけない（must) 行くと　決めなければ　いけない。 You must make your mind to go. ( seems weird to me, could you correct this for me?) 行かないと　いけない。 帰らないと　いけない。 買わないと　いけない。 勉強しないと　いけない。 結婚しないと　いけない。 全部 must to do です。

As Miho-san answered, it means "must go" and we have several ways of saying "must + verb" depending on the person. For example, "must go" (formal form) 行かなければいけません いかなければなりません 行かなくてはいけません 行かなくてはなりません There are some more but you don't have to memorize all. I wrote them to be able to understand when you hear one of them. Please get accustomed to it little by little. I hope this doesn't confuse you.