first, you should read some introduction to symmetric cryptography.
otherwise it is likely that we make severe mistakes.
in Java, you can do the encryption using the javax.crypto.Cipher class. e.g

byte[] iv = new byte[16]; // iv...initialization vector
SecureRandom random = new SecureRandom();
random.nextBytes(iv);
IvParameterSpec ivSpec = new IvParameterSpec(iv);

byte[] keyBytes = ... // 16 key bytes, either you "know" it or you generate
e.g. using randomly like the IV

SecretKey secretKey = SecretKeySpec(keyBytes, "AES");

Cipher aes = Cipher.getInstance("AES/CBC/PKCS5Padding");
aes.init(Cipher.ENCRYPT_MODE, secretKey, ivSpec);

byte[] ciphertext = aes.doFinal(plaintextBytes);

then write to file the "iv" (initialization vector) followed by
"ciphertext". however, you should implement this using streams rather then
just one big byte array as shown above.
to do the reverse, simply read the first 16 bytes and take them as IV. the
rest can be decrypted in a similar manner; e.g.

Cipher aes = Cipher.getInstance("AES/CBC/PKCS5Padding");
aes.init(Cipher.DECRYPT_MODE, secretKey, ivSpec);

where the key must be the same as at the encryption side (the IV either, but
the IV need not be kept secure).
see also
http://java.sun.com/j2se/1.4.2/docs/guide/security/jce/JCERefGuide.html#SimpleEncrEx.

i hope this helps for starting.
note that old Java version do not support AES per default. thus you may use
SUN Java 1.4.2, which comes with AES. if you have to support older versions
of Java, you may use a JCE provider; e.g. IAIK-JCE
(http://jce.iaik.tugraz.at/products/01_jce/index.php), which is very good
;-).

regards

 Karl