Nothing beats match experience, get some matches under your belt, even if it's at a lower level and build up from there. I've seen players look sensational in the park, and even in training sessions, yet out on the pitch like you said they make bad decisions - when your playing at a high level, the tempo is fast and so decisions needs to be instantaneous and natural, that is why experience is a must.
As far as making a team in concerned, be vocal, get yourself noticed. Bad players get in ahead of better players because they make themselves heard by organizing the midfield/defence.
Competitive games are so much different to casual games, it's all about mentality. Also, stick to one position for now, master it. Know the spaces to move into and the passes to distribute off by heart, you don't want to be the guy that has to fill in all the open positions, pick the one you are best at and stick to it. Experience won't do you much good if you're at RB one day and CM the next, you'll learn one thing and forget the other. You learn best by repetition and consistency.
Hope some of this helps! Good luck