It is such a big advantage to get your first serve in play.
Your opponent has no idea how you are going to hit the first serve-speed, spin, direction, etc. If you miss your first serve, you have to get the second serve in. This means you have to give yourself more margin for error and it is likely your opponent will begin to figure out your tendencies under the pressure of a second serve. The returner of serve will tend to be more aggressive on a second serve and a good player will return the ball with the intent of getting the opponent in trouble.
A good serving idea is to hit the serve aggressively to the middle of the box as a starting point. As the server gets more confident, dividing the box in half and hitting to one side or the other is a good way to be aggressive with a good margin for error.
Another good idea is to hit a first serve more like you would hit your second serve if you have just played a point after missing your first serve or double faulting. This will keep you from going several points in a row without making a first serve.
Remember, a first serve hit to a good spot will help get the point going in a positive direction and allow the server to keep an advantage over their opponent.