When there are two roofs involved, I would recommend explicitly selecting the wall and roof, before invoking the Trim to Roof command. This will eliminate any doubt - ArchiCAD will make assumptions.
As for the Split command, you'll get the hang of it. Without looking back at the tutorial, all I can say is try and be as flexible as possible, slow down a little (you can't expect to be as fast in any new software as you were in your previous one), know that ArchiCAD can offer many different ways to do the same thing so just learn one that works for you and stick with it for now, expect things to go wrong at the beginning.
As for training, what is your budget set at? Seriously, it makes a big difference - you get what you pay for. Any kind of training should be considered an investment, and when it comes to a software that you will be using nearly every day of your life, quite an important investment.
Cheers,
Link.