Where do i start?

I bought a journal with 3 separate notebooks half a year ago and I realised that was not for me. The journal was too big to bring along and I just want one thing for all my thoughts. Are there any notebooks you guys would recommend?

I was also wondering which one of you has an index with a colour coding system. I tried it, but I found it difficult to actually put my interests in a category if you know what I mean. Any tips for this are welcome as well, otherwise I might just scrap that part.