Do I need to prepare plain text before encryption? When I want to encrypt chat messages (in ASCII) before sending, whether it brings some bit of security if I add some extra bytes to the message or shuffle it in some way? I can even try to make data to look like uniform distribution.

Because chat messages consist only of letters and even words, but don't contain binary data and other bytes except that from ASCII, so it gives some information about unencrypted data to eavesdropper.

But I'm afraid that some manipulations over the text could bring much more information to the attacker than doing nothing.
If you are using a modern, secure cipher, there is no reason whatsoever to perform such manipulations of the plaintext.

I'm not sure how to elaborate much more on the topic than that. The entire purpose of a cipher is to perform the exact types of confusion and diffusion you describe. Only, the approach that it will use has been designed, peer-reviewed, and cryptanalyzed by professional cryptographers. Your hypothetical algorithm has not.

At best case, it will do nothing to improve the security of your ciphertexts. At worst case, you can introduce bugs that unintentionally cause plaintexts to be unrecoverable from the ciphertext, or ones that accidentally reveal information about the plaintext (for instance, via a leak of timing information).