It has been suggested that that the first thing you do with a book is write the last line of that book. Some feel that is kind of like spoiling the fun. Ideas come from different places. The goal is to start with something and just go with it. Try it both ways and see which you like.
The thing is you learn little by little, so sample different genres to see where you enjoy writing. If you want to be a writer, you really only have to do two things you have to read a lot and you have to write a lot, and you have to continue to get excited by what you are doing. Remember writing is subjective, sometimes you get bad advice on good work and sometimes you get good feedback on bad work. The thing that makes it worthwhile is that people take writing seriously. Dreams are free and that is a good, so write, have fun and continue to be creative with your words.