Make sure your toes are in a straight line from the middle stump to your dominant feet. The bowler tries to aim at your wickets and so stand in front of them while covering half of the wicket to protect it from falling.
If you want to practice your swing, pitch a tennis ball right in front of you and hit it with full swings. This will make your swings better. Doing the swing will make your bottom hand grip better as your dominant hand will be on the bat.
Being nervous during the game can throw you out of focus and off your game. You will tend to make mistakes while nervous. Always practice regulating your breathing and visualize the trajectory of the ball. Try to block out all the excitement and nervousness coming from others and concentrate on the ball.
You can refine your skills by following through with your swing. A good follow-through is critical. Make sure you follow through your bat for the whole swing even if you miss the ball. This is done to hit the ball as far as possible. Keep practicing so that even if you miss the ball, you do not pop the ball in the air and get out easily.