Terrorist (Bomb scenario) - You gotta plant the bomb before the time runs out. Try hiding bomb so CT will have notime to defuse it. The best tactic is that team splits into 2 groups and goes from 2 sides if one group is under attack second group will come from another side and clear the site from CT. Once you are alive alone and there is a full team of CT then... just play time. Give CT as few seconds as possible. if CT have under 5 seconds (DEFUSE KIT) You won the round. (under 10 seconds if defuse kit is missing. If you got killed right after planting bomb by 2 CT then just hope that when one starts defusing bomb another kills defuser and gets cooldown.
Terrorists ( Hostage rescue) - You gotta elemenate whole CT team or run donw the clock so CT won't have chance to rescue the hostage. If they got the hostage, then hope that they won't reach the rescue zone in time.
Use aim practice maps and surf maps to train for better moving and aiming and shooting.
REMEMBER! DO NOT GO IN COMPETITIVE MATCHMAKING BEFORE YOU ARE GOOD ENOUGH!