How do I get into Mythics?

Hi all,I’ve been playing WoW on and off for years since release and been enjoying TWW for the last couple of months and recently started playing more seriously in this current expansion.

Currently my main is a BM Hunter with item lvl 630 but also been playing as Retro Pala with item lvl 619 (I believe). Now I wanted to try mythic+ keystone dungeons and have watched/read a lot of guides but still don't now how or where to start exactly.

Can you give me tips? How do I actually start, in the LFG tool or?

I also read comments that a lot of people get declined from groups and almost never accepted. What information do I need to tell me when applying for different groups? Do I need to show my gear as well?

Thanks in advance!