Just yell at it, it won't break! Eventually, you'll remember the ones you most want to use and that get you the result you want, but don't be afraid to experiment, it can only say 'Hm, I don't think I can find the answer to your question'
There's actually another thread about the difficulty of remembering all the different skills so you can ask the Echo to open the one you need, and it's a big issue. Hopefully, Amazon will get that sorted eventually so that, when you ask for train times, it knows which default skill to open and interrogate. Meanwhile, just get used to throwing questions at it and seeing what comes up.