I remember that the first thing I did was after I finished the tutorials, I went into sandbox mode and just experimented. I recommend starting off by attempting to get into a suborbital flight with a rocket of your own design or try to recreate some things that the tutorial section showed you, simple missions. If it fails, try to figure out what went wrong and improve it. If you are having trouble doing something on your own then go check out some up to date tutorials. I would recommend Scott Manley, he not only explains game mechanics but he also relates in-game events to real science but sometimes he gets really sciencey in some of his tutorials which might confuse you.
As some other people already said, career mode might also be a good way to start off because you begin with very few parts and eventually unlock more and this way you learn to not make crazy rockets since you have to be mindful of your resources. Career mode also provides you with contracts which give you different missions so you have a goal, something to work towards instead of just wondering what to do and blowing rockets up.
Good luck!