i started playing poe last week and i want to use my fg but i dont understand what im suppose to do. i come from 20 years of d2

Majority of the ppl here use fg to buy currencies.
Some ppl use fg to buy services.

Early in a league, within 2 weeks of league start, people mostly buy divine orbs and chaos orbs.
Mid to late in a league, people mostly buy divine orbs and mirrors.

ok but where i am lost is i was asked to put a item in a trade chat for the value of the divs i wanted and i am totally lost on what that means

you gotta put something to avoid getting detected

list item in your stash for 100div > poe trade site filter your account name > give the seller the URL to your item >seller will click direct whisper to act like he/she is buying your item like normal

RMT will get you banned in POE in case you didn't already know

If you purchase a "premium stash tab" from GGG, it adds an extra stab into your stash. The premium stash tab can be set to public. When you place an item into the premium stash tab that is set to public, you can put a price on that item so that when other people are searching item with similar affixes or that exact same item, your item comes up as a result. If the price is right, then people will pm you in-game and buy it from you.

When it comes to trading fg for divs, ppl just put a random item and set a big price on it that no random person would find it through search and be insane enough to just buy it.