Powerliftingwatch.com has all levels of meets listed from local to national level. Just find a meet that is in your area. Trust me, it doesn’t matter how it’s listed, if there is no qualifying totals to enter, then you will see people from rank beginnners all the way up to high level athletes at any given meet.
Powerliftingwatch has a page called Compare Federations that will tell you what is allowed in each federation, but if you just search for meets in your area there is usually a link to the entry form which will tell you what divisions there are going to be (i.e. Raw/Equipped, Full Power/Push Pull/BOF, and age brackets).
You would compete in the Open class since you are too old for Juniors and too young for Masters. And unless you plan on using a shirt/suit/briefs, you would compete in the Raw division. Each federation allows different levels of gear (belt, wrist wraps, knee sleeves), even in the raw division so you have to check with that fed’s home page or check the Compare Federations page on plwatch.
You can usually register for the meet on the day of the meet, but it’s more expensive that way, so if you know that you are going to compete, then register early. As far as joining the federation and paying for it, you can just do that on the day of the meet, and I’m pretty sure the cost is the same whether you do it in advance or not.
Also, you will need to select your weight class. Just choose 181 on the registration form. You will weigh in anywhere from 24 hours to 2 hours before the meet, depending on the fed. If you go over your weight class, they will just bump you up to the 198’s, so it’s not a big deal if you pick 181 and come in at 185, or vice versa.