How do you trade items in Team Fortress 2?

When you have someone on the server you want to trade with. Press escape to open the pause menu and click manage items. Click trade items or something similar. click the middle button and choose the person you want to trade with. You'll send them a request for trade. If they accept the trade menu will open, put the items you want to trade in one of your trade slots, check the "ready" checkbox when you're ready to trade. Then choose trade now at the bottom of the screen.