TF2 trading has been going for a long time so it's quite mature. Unlike what some people said, you can get old effects with new crates but only with Keyless Cosmetic Crates, otherwise yeah it's uncrate a given series. Read What you can (and cannot) find in Keyless Cosmetic Crates and TF2 Unusual Wiki is the best guide.
First of all, re: Crates. Unboxing crates is gambling. Straight up. Only unbox for fun. There is almost no chance you would make a profit. Only unbox if you really want to and have extra cash that you are going to instead throw into the street in exchange for used cigarette butts and you don't care if you get butts or gravel.
Okay that said. Trading can be fun and since it is mature there are a few guides that are helpful in two ways:
- Guides for trading
- Guides for not getting scammed
Both types are important to read up on. Don't get scammed. Discord is AWFUL for getting scammed. Anyone can impersonate anyone. Only discuss and execute a trade on steam. If someone wants to DM you on discord to talk a trade. Politely ask them to add you on steam and discuss it there. If they decline. Likely a scammer.
Good starting out guides:
Beginner's Guide to TF2 Trading
All in 1 Guide - Trading Basics + Scrap to Unusual + All Kinds of Scams.: Just a good starter's guide.
Current Scams And How To Avoid Them: old but still relevant
Advanced guides relevant to understanding items and rarity.
TF2 Statistics Guide: Unboxing and item stats pertaining to chances to get stuff that people are interested in.
Rare Items in Team Fortress 2