Fault
For a given play, there is only one service unless there is a "let" (ball touches the net or opponent not ready). A failed service is a point for the opponent. This is different from tennis where a service fault allows another serve.
You can hit an underhand serve in tennis; however, the overhand serve is more effective because it is more powerful. That is why you don't see the underhand serve used much.
a serve
Badly.
A tennis ball!
It depends on how hard you want to hit the ball
a tennis ball
Let
Yes
A tennis ball.
a fault