First of all, you have to start from your passion. (What do you like most where on the process of creating a blog, you will not get bored as you progress in your goal. Second, consider the demand and competition. This is also debatable. Because it doesn't mean its not in-demand, there is less competition. So, best idea is to search for keywords related to your niche and analyze which is profitable niche. Targeting in-demand has high competition. If you are just starting out, better target those in-demand with less competition so to rank immediately (not quickly). I suggest you use a software to analyse the keyword competition. There are plenty out there. (Google it!). Also, keywords are relevant to rank and get notice, but still you have to satisfy the readers and establish your credibility first. (There is no overnight success in creating a successful blog). You have to work it out. In any niche, your dedication matters. Reach out other bloggers, do guest post, comment, etc. Be active in social media, to name a few that you need to do. Looking forward to your success!