A VB.Net beginner Problem

Hi,
I am a vb.net beginner from another country. So my English speaking is too bad.
My company is doing a project about RSA encryption in VB.net. I know a basic using about RSACryptoServiceProvider class. But i can only to use the random Public key and Private Key by RSA Object in VB.
For example,I already have a pair of keys(private key and public key).That keys are related(The keys were generate by another program).
Now I want to use my public key to encrypt my String and use my private key to decrypt my String. But I don't know how to import my key to RSA Object(Use FromXmlString/ToXmlString).

Please Help Me.(My english is too bad. Please use simple English)