Assuming you are playing the original Counter-Strike: Source map, here are some tips:
-Start off by buying an AWP and a good shotgun like the M3. You will need the AWP for long-range battles and the shotgun for close quarters.
-You will also want to buy some grenades, smoke grenades are particularly useful on this map.
-Try to stay on the high ground as much as possible, there are a lot of good sniper spots on this map.
-If you can, try to get control of the bridge area, this will give you a good vantage point over a lot of the map.
-Be aware of the fact that there are a lot of places where enemies can hide and ambush you, so always be on the lookout.
Hopefully these tips will help you out, good luck!