There are for algorithms of encryption/decryption with in the .Net framework

DES algorithm, RC2 algorithm, Rijindael algorithm and TripleDES algorithm they are represented as classes

First step is to include directive of cryptography
witch is System.Security.Cryptography

Second step is to create your own key, to do that
string password = // you can put here your password
UnicodeEncoding objectEncoding= new UnicodeEncoding();
byte[] key = objectEncoding.GetBytes(password);

third step is to create a stream where the encipted data will be putted

FileStream CryptedFile=new FileStream(cryptFile, FileMode.Create);

encrypt the contents

RijndaelManaged Crpt= new RijndaelManaged();
CryptoStream Encryptor= new CryptoStream(cryptedFile, Crpt.CreateEncryptor(key, key), CryptoStreamMode.Write);

And the the data encryption is done according to the Rajindael algorythm
u can use another algorithms like the DES or u can develop your own encryption algorithm