There is no any way to encrypt something on client side without danger that encryption method could be reverse-engineered.
(ok, there is one method surely - usage of hardware keys, but for most types of web-applications it is obviously unusable)
But if you want data to be encrypted on client and decrypted on server, it would not do. One of usual ways to deal with it is using https (ssl) access to your resource instead of http.
However, you'd better at first try to explain (for yourself and for other peoples) the following questions considering security:
1) Which kind of data you are sending to server.
2) Which kind of attacks from third persons or cheating behavior of your users you expect, regarding different parts of data mentioned in answer to first question.
3) What size of profit could potential hacker gain from breaking your security.
I know that sometimes (often) people are trying to protect themselves from attacks which are unlikely to happen and forget some more simple issues which render planned protection measures useless.
You may also look at protection schemes used by online banking resources or billing systems. There are different variants with sending confirmation codes, passwords or encryption keys by e-mail or SMS, etc. Anyway it all depends on answers to mentioned 3 questions...