Capitals are used to shout something in on line speech. For example, If I were angry I would write,

LEAVE ME ALONE! or some such thing.

So, don't write in all capitals. Use capitals like they are supposed to be used: at the beginning of sentences and with proper names. I would never use all caps when speaking to a Dom.

Many Dommes and Doms like to capitalize the first letter of their name and use a small letter for the sub's name. This is not really important to all doms, especially one who is serious and not just playing dom.

As far as how to speak to your Domme, well as a submissive I would start by being polite and honest about my thoughts and feelings. I would let the Dom guide me. I would ask questions so I could learn how to better serve him/her, always being sure that it was ok with him first, of course. A good Dom/Domme will guide you and answer your questions as you learn to serve him/her.