1) Tap on the Lobby button on the Homescreen.

2) Select Public match.

3) Select any game mode(Team Deathmatch, Deathmatch or Bomb Defusal) by tapping on the gear icon on the top right.  

4) Tap on Play to start the match.

OR Else

1) Tap on 'PLAY' button on the homescreen.

2) The game will Quick match you with opponents on any random map/mode.

You can also browse the currently running matches by tapping on Browse.