Go here, this is the official trade site. Or PoE trade it doesn't matter.
Trade - Path of Exile
find what you want, click tontact and it will copy to your clipboard or put up a message for you to copy to your clipboard. In game send whisper with paste and it will send the message to that person. If they aren't afk they will send a party invite, join party, right click their picture, go to hideout. When at hideout they will starr trade, hit accept, they will stick it in trade window. Put what you're trading, and hover over what you are getting and hit OK/accept or whatever it is. That's it, now you can go back to your hideout or wherever using the waypoint. You're done. Remember it won't let you finish trading till you hover over what you are getting, it will do the same for them.
Searching for stuff fill out the part where you list your lvl and str/dex/int as max so you don't buy something you can't use yet.