Use "as long as..." to say that something is OK, but only under certain conditions. For example:

A: Can you come out with us?

B: OK, I'll go, as long as you promise not to make me dance.

This means that Person B will go out with Person A, only if Person A does not make Person B dance.

"As long as" can often be replaced with "if":

I don't really mind dating a smoker, as long as they don't smoke indoors.

I don't really mind dating a smoker if they don't smoke indoors.

But there are a lot of cases where "if" is OK but "as long as" isn't:

If you want something to drink, go get it yourself!

I'm not paying him if he doesn't do a good job.