I need to look posh?

No you don't. Just be yourself. No acting necessary.

Go to a mens clothing store where you can have a clerk wait on you. Tell him you are going to a party where you need to look posh and have nothing in your wardrobe that is really posh. Then let the clerk help you put together a suitable outfit. Be prepared to spend a few hundred dollars but if this is the lifestyle you will be living as a man you will get your money's worth out of the clothes. Also do your shopping far enough in advance of the party to allow time for any tailoring that may be needed.

Look at gq magazine or internatioanl male.

Gray or black slacks - no jeans or carpenter pants! Chinos would be OK to. Have the pants be around your waste - no boxers showing!

Dress shoes. Wear a belt. The belt color must be the same as the shoes. Your shoes should be polished.

Dress shirt - buttoned down. Put on a tie or bow tie - you can get them pretied in advance - but its better to learn to do it. If you want to wear red or green and white because that is the holiday season for the shirt and tie - thats fine too.

Wear either a sweater or blazer. Navy blue is a good choice color for a blazer - it goes with a lot of things.

A dab of cologne never hurts!

Do some research on how a button up shirt should fit properly, and go find one that does. (You may have to have it tailored, there's no shame in that.) Some nice dress pants and a well-fitted sports coat with a bow tie or straight tie will complete the look. You can mostly just research men's fashion.

nice pants, button front shirt, maybe a sport jacket?

Wear a posh suit with a posh tie and some posh shoes.

https://www.youtube.com/watch?v=5eeex5Px1WU

what does posh mean

Just wear a white collared dress shirt beneath a nice, simple sweater (preferably burgundy, deep green, grey-ish navy, or brown) and slim black dress pants. Add a tie or bow tie for more affect and maybe a watch on your wrist as well.

Tighter khaki jeans and a button up shirt with a simple pattern and light color. Black vans/shoes and (try to) gel your hair to the side. Heck yeah. Hope this helps man.