[Warframe Slots And Inventory]

I try to trade but nothing really happens. I'm sortta new and I'm pretty far ahead on in the game with oberon and equiped with paris, dex furis, and orthos. I would really like some more slots to put my weapons and warframe in but im still confused of how to trade. Do you like talk to people to trade or do you have to like go to your mod section and sel it from there? so confused plz help :(