If you're not familiar with the map, use your radar and follow a large bunch of where your team goes.
Don't be the first to rush into new areas. Always follow teammates and back them up instead, that way:
1) Your teammates can absorb the enemies bullets.
2) You can pick off the enemies shooting at your teammates.
3) Chances are the enemies will not be at full health, making them easier to kill.
Rinse and repeat until you get a good feel for the map and the critical points.
If you do this, though, do not throw flashbangs. You never know when your teammates decide to corner.
Good luck and have fun! Just remember there's a reason why CS:S is 50 years old and still as popular as ever!