Atmosphere sets the mood of your thread. The way I think about is, is like a film, you want to describe as much as you can. So firstly set the scene, where are you? what’s around you? what are you wearing? That kinda stuff. then you can move onto the actual purpose of the rp, whether it be meeting up with someone or fighting someone.
Remember the smallest details matter, it’s what can make or break a good quality post. Another thing is to incorporate your characters sense into the post. The smell, sound, sight, touch and taste (if applicable.)
You can describe what you care feeling, and from that can come ticks in your character, ie if your nervous, you may sweat or something along those lines.
Just remember, you can’t describe anything about anyone else, only you, and maybe an NPC if you have one in the thread. The moment you start describing how someone else looks or feels it becomes godmodding, which is frowned upon. The only exception to the rule is if you have permission from the other character.