There are a total of 11 Agents currently available in Volarant. The way you can unlock these is by completing them. Follow this guide and you will be able to unlock agents and free weapons in Valorant. 
If you are new, you can easily Unlock 2 Free Agents just by playing a few matches. After that, you have to complete each of the agent contracts and reach level 5 of them. The fastest way to level up agent contracts is by completing Daily/Weekly Missions! 
Weekly missions give massive amounts of XP. (Around 13,000 – 18,000), you get 3 per week!
Daily Missions give you around 2000~4000 XP, you get 3 per day and are very easy to complete. You can complete these missions in any game mode, Spike Rush, Unrated or Competitive. 
You can also earn XP, just by playing more games and winning more rounds. (Any game mode). Spike Rush mode gives you around 500 – 15000 XP, depending on the result of the game.
Unrated or Competitive mode gives you around 25000~5000 XP, depending on the result of the game.
How to unlock a free weapon?
There is a free weapon skin related to every agent contract. 
But you need to fully unlock a contract to earn a free skin! (Reach level 10) which is a lot of grinding. Keep completing Daily/Weekly Missions. Keep grinding the game and add these to your collection 