Clearly you need to pad the plaintext before encrypting it.
Pad it to some fixed size – optimally this should also be a multiple of the block size (16 bytes are nowadays standard for block ciphers), so you don't have to pad again for encryption.
Make the padding removable. There are several methods of doing this:
If you now that your message can't contain certain bytes or byte sequences, you can use a separator. A zero byte works if your plaintext contains only printable text in ASCII or some compatible encoding.
This will not work if the plaintext can contain arbitrary binary data, if you don't escape it first. Not generally recommended.
If your message has a certain internal structure which tells you when it is finished, you don't need a separator – simply parse it and stop when the message ends.
Here you'll have to pay attention to buffer overruns and similar. Not generally recommended.
Encode the length of the message at the start of the message. (Of course, this only works when you have the whole message before starting to encrypt, but for your short messages this seems doable.)
This is also quite easy to implement.
Encode the length of the message or the padding at the end of the padding. A popular version is the padding used in PKCS #5, which indicates the padding size repeatedly in all the bytes of the padding. (This limits you to maximally 255 bytes of padding, though.) You can use a similar scheme, indicating the padding size with two bytes.
This has the advantage to be easy to strip off.
Have the padding of some fixed format. Rossum's bit padding variant is this: add one one bit, then fill with zeros.
This makes you parse the message from the end to find the start of the padding. Often backwards parsing is more complicated.
Whatever you do, the actual padding can then be either random, zero, or arbitrary (of course not for the versions with fixed padding format).
Then encrypt the message using a secure encryption algorithm and mode of operation (do not use ECB mode – it is generally broken, and even more in the case of some empty or otherwise fixed padding when you want to hide the size).
Then add authentication (a MAC such like HMAC), of the whole encrypted message. The receiver should reject the message if the MAC is wrong, and not even start decrypting then. Doing otherwise can give an padding oracle when an invalid padding is detected after decrypting, which can lead to loss of confidentiality.