Cherche et tu trouveras, demande et tu recevras.
However, this phrase isn't commonly used in French. Here are some more natural-sounding alternatives:
* Qui cherche trouve. (Whoever seeks finds.)
* Demande, et on te donnera. (Ask, and you will be given.)
* Si tu veux obtenir quelque chose, il faut le demander. (If you want to get something, you have to ask for it.)
These phrases capture the essence of the original saying, while using more natural French.