Crypting data

Hello

i have some simple questions on crypting.
I like to encrypt some data i send to some customer and i'd like to
ensure no one is able to create/encrypt the data except me.
I thought of having 2 keys that are used to encrypt the data and only
on is send to the customer. That one is used to decrypt the data. But
without the second key its not possible to create/crypt the data.

Can anyone give me some hint what kind of algorythm i have to search
for? I need to implement it on a dot-net 2.0 application.

I'm very new to crypting so please forgive my ignorance.

thanks

.