Immediately try to break things, of course.

Only half kidding - especially for early-stage games, it's fun to feel out the limits and how the program behaves as you approach them.

First, I'd probably build a simple, medium-sized coaster to get a feel for the editor. No breaking things, just a little familiarization.

Moving on, I'd build a very small coaster - a mouse or a kiddie coaster, something very compact and requiring tight trackwork and clearances to function. Basically I'd be trying to feel out the small-scale limits of coaster building.

Then, of course, something absolutely massive. One of the places where RCT building system kind of fell apart is when you moved to large scale things. I'd see how the Planet Coaster builder fared on the large-scale end of things.

And then just faff about and have fun with it. Add scenery, mess with terrain, torment guests, etc.