Please help a complete noob with deniable encryption!
If you aren't concerned with integrity guarantees, one easy way of accomplishing this is via AES-CTR encryption. For a given ciphertext, decrypting with any AES key results in a valid plaintext.

That being said, you should be concerned with integrity guarantees. I'm not recommending deploying any crypto which does not meet standard security notions like IND-CCA2.