You'll always have things on your mind that you know you should get around to eventually. These include building a website or a Facebook page, getting some sort of equipment, placing ads, etc. The number of things you need to do can seem overwhelming and easily lead to procrastination.
Begin it now
To prevent this from happening it's important to get into the habit of regularly starting something new -- no matter how small. Do this, and the next step is easier. Also, your subconscious will be working on things. You'll come back to that project armed with new ideas and approaches. You'll "hit the ground running" on it.
This applies to all tasks required, but especially to an online project such as building a new site. This is because time is a significant factor in SEO. The longer a URL is up there, the better. And it's a process that will inevitably develop in stages. So you should get the first one underway soon.
That said, you don't want to rush into it. You should give a lot of consideration to the keywords you'll be targeting and the content you'll include. But once you've done that and bought your domain name it's a good idea to start your site ASAP. What you build doesn't have to be final. It's just really important to just put some stuff up there. The fact that you've "gotten the ball rolling" helps you psychologically as much as anything else.
A lot of people make the mistake of believing that a website should be made perfectly and completely from the start, and then just left up there. But the opposite is true. It should be a work in progress -- even if you don't have a blog that you're continually updating. You can always tweak pages, add new ones, etc. That's why the sooner you start, the better.