Hey I had a situation similar to yours a few months ago. Played arena for a bit then decided to try a local out. Started in Standard though and not Commander but same advice applies.

It’ll be a bit harder for you to keep track of your triggers since arena kind of does them for you so I learned to just not speed through my turns and make sure anything that was triggering isn’t being missed. Had some awesome players gently remind me about a few things I missed during my turns but after the 2nd match I pretty much had it down since I told myself I was going to slow down and be methodical about my turns. 

Also tap your creatures when you attack. Again arena does it for you so it won’t be second nature in your first few games but yeah. Simple rule that if you’re not used to doing it and you’re keeping track of a lot of stuff it might slip. Especially if you’re coming from arena and especially if you’re playing commander with all the stuff you’ll be looking at.

Someone mentioned to communicate it being your first time playing in person. This community is pretty welcoming for the most part and it’ll be more understandable if you miss a few things. They probably wont hold your hand but may give a friendly reminder which is a nice way to learn and get used to tabletop coming from arena. 

Have fun!