I would begin by simply hand-building your rides at first. Produce a ride that you've constructed by hand using real life coasters as a reference for things like shaping, simply by eye. Release it on the exchange here and have people rate it and give you tips on where you can improve. After you've put a few rides on the exchange over a course of weeks, months or years, you will have built up a good knowledge (from people's ratings) of how to contsruct a realistic coaster.
Then, if you want to take the next step towards realism, move into using the tools and learning how to use those to turn a rollercoaster that is realistic in terms of the g's, layout, style etc... into a coaster that's completely realistic in terms of it's construction and trackwork also.
Some people are naturally gifted with this sort of stuff, but as said above, your first coaster isn't going to be amazing, the only way to achieve that is with practise and trial and error.
Hope all of that made some sort of sense!