You get to expand your inventory once at the start and the game gives you this option free. Then you have your bank slots where you can expand the slots with ingame money. I'm lvl 38 and I only had issues with inventory during my first hours but I then realized how to solve it. I move important stuff to the bank that I might need for later. I sell things that can't be sold in marketplace to merchants and I sell the rest to the marketplace. I also upgrade my weapon and everything else a lot so that also helps me clean out inventory.
You need to stop and think a bit about how to solv