hoanglinhbiotech: I want to try some game. How can I try?
Create an account (which you already have done), go to browse games in the store.
Select "FREE" for price, and pick one, or pick all.
Some of those free games are full games which are distributed as freeware, but a couple of them are free demos of games that you can purchase if you like the demo.
Additionally, there are some limited free promos, and GOG Connect which allows you to claim some games which you already own on Steam.
There are also some giveaways on the forums where people give out extra codes for games they already have etc.
If you want to try a specific game, as in "try before you buy", there's no way to do that. Legally, at least.