I suppose we really need to know the sort of thing you are thinking about - a shop, some sort of service or whatever? What you are planning will determine whether you are starting too big imo.
As for which site building software, I presume that you are going to build the site yourself. I have only experience of having one built so I can't really help you there but I just wanted to clarify what it is you are trying to achieve.
As for why you can't get started? Well, it is a lot of work and it will take a lot of time so it will be a commitment. Better not to rush into it until you are absolutely sure as there are costs attached.
If you're asking other people to answer your questions and resolve your - as yet non-existent business - problems, clearly you're not ready to startup.
Sorry - harsh but true. The only person who can make those decisions is you. You'll get some stuff wrong and some stuff right. Then you get more right than you do wrong. It's called learning and progress.
I procrastinated when I decided to go self-employed - in my case, it was partly about making sure everything was in place so that nothing would go wrong, which is unrealistic. It was also about not wanting to spend any money (eg on professional indemnity insurance) in case I didn't earn any money, then not being able to "launch" because I didn't have insurance - a kind of self-imposed chicken and egg situation.
I think the underlying issue was fear - fear of failing, and fear of losing money.