You usually use "しなければならない" when you are required to do something by social rules such as law and religion. On the other hand, you use "しなければいけない" when you just have to do something for some reasons including your personal ones.   

日本で暮らす以上は日本の法律を守らなければならない。

会社の帰りにスーパーで買い物をしなければいけない。

I'd say, however, that you don't have to worry too much about the usage of "ならない" and "いけない" because even native speakers of Japanese don't. These sentences above would make sense even if you should interchange the two.