No matter how many years you have been playing tennis, you will ALWAYS be working on both your first and second serves.
Starting out, work on second serves, because you WILL miss most of your first serves for more than 5 years. That way, at least you second has a chance of finding it's way into the court.
You will NEVER be satisfied with EITHER your first or second serves, because you can never practice them enough.
Why? You ask?
1st serves.....flats up the middle, out wide, into the body.
Top/slices out wide, into the body.
Twists out wide the other side, into the body.
That's at least SEVEN different locations and different spins, for first serves alone.