French lines up its if sentences by a strict sequence of tenses, and the first pattern is the easiest. When something may really happen, si takes the present, and the other half takes the future, the present or an imperative:

Si + present	Result	Example
si + present	future	Si tu viens, on ira ensemble. — If you come, we'll go together.
si + present	present	Si tu veux, je t'aide. — If you like, I'll help you.
si + present	imperative	Si tu es fatiguée, repose-toi. — If you're tired, have a rest.

Just as in English, the future never comes straight after this si: si tu viendras is as wrong as if you will come. (Si meaning whether is another matter: Je ne sais pas s'il viendra. — I don't know whether he'll come.)

The halves can swap places, and then you don't need a comma: On ira ensemble si tu viens.