TL;DR By combining trading cards to badges.
First of all your games are not stored in your inventory, so this amount will not be touched. Exception: If you add them as presents, every present is one item.
If you still need more place in your inventory (maybe because you collect a huge amount of ingame items from Team Fortress 2, Dota 2, CS:GO and so on), you have to collect Steam Trading Cards.
Nowadays many games do have trading cards available. After an amount of playing time of a game (mostly about 30-60 minutes) you get 1 trading card. In the most cases you can't get more then [Half of all trading cards]+1 for one game, for which you have to trade with others.
Another option would be buying them on the Steam Communiy Market. You can buy from any game, as creating badges and collecting cards is NOT limited to games you own.
If you have a complete set of trading cards, you can then create a so-called badge out of them. You can look at your progress of collecting cards and create badges using the following site:
Just replace the ID with your own.
If you create one of those badges, you mostly get:
- Experience points
- One smiley of that game
- One community background of that game
- Sometimes a coupon for another game
With that Experience Points, you can increase in level. In the first levels one badge means one level. And every level your inventory size is increased.