Let
Do you mean "let"? If so, a let is when a serve touches/nicks the net AND lands in on a player's serve. People call "let" so the server knows their serve hit the net. If a server hits a let, they get to redo their serve.
I am not sure but just call your local tennis place
Losing your serve is when you lose the game in which you were serving, it is said the oppenent (whoever that may be) broke someones (whoever was servings) serve. If you win the game then you held your serve.
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.
Badly.
a serve
A tennis ball!
because you get a second serve if you mess it up and if you mess up both serves the other opponent gets the point
It depends on how hard you want to hit the ball
a tennis ball