Which episode does Gintama get good?

As what was said, the very first serious arc will start at episode 58. I felt the same as you during the first episodes of Gintama, and as you reach that first serious arc, it will make you want more! As serious as they may be, they contain a good balance of humor, action and drama.

When did Gintama start getting good?

To me it started getting funnier around episode 14, I think the second half of the series 100+ was funniest to me, though it’s always pretty funny. So I say stick with it.