How to convert string key to byte[]

public int   EncryptFile(string inputFile, string  outputfile, byte[]  rijnKey, byte[] rijnIV)
		{
			try {
				
				FileStream fin = new FileStream(inputFile, FileMode.Open, FileAccess.Read);
				FileStream fout = new FileStream(outputfile, FileMode.OpenOrCreate, FileAccess.Write);
				fout.SetLength(0);
				
				long rdlen = 0;              
				long totlen = fin.Length;    
				int len;       			
				SymmetricAlgorithm rijn = SymmetricAlgorithm.Create(); 
				CryptoStream encStream = new CryptoStream(fout, rijn.CreateEncryptor(rijnKey,rijnIV), CryptoStreamMode.Write);
				int[] digits = terdigits(Convert.ToInt32(totlen));
				int length = 1;
				for (int i = 0; i < digits.Length; i++) {
					switch (i) {
						case 0:
							length = digits[0] * 1000;
							break;
						case 1:
							length = digits[1] * 100;
							break;
						case 2:
							blength = digits[2] * 10;
							break;
						
					}
					if (length != 0) {
						byte[] bin = new byte[length];
						len = fin.Read(bin, 0, length);
						encStream.Write(bin, 0, len);
						rdlen = rdlen + len;
						len = 0;
					}
				}
				encStream.Close();
				fout.Close();
				fin.Close();
				return 1;
			} catch (Exception e) {
				return 0;
			}
		}

result = a.EncryptFile(SelectedPath + @"\" + FileName, TargetFilePath, key[0, 1].ToString(),iv);