It all depends on your budget and if it is something you want to do by yourself.
If you are just starting off, then look at
SEO first. Work on your main pages/categories and make sure your website is user friendly and easily accessible to the search engines. One thing that is inexpensive is time.
Ecommerce stores take a lot of time to get right, and a lot of effort to maintain. The rewards are amazing though once you are happy with how it functions. Make sure all your on-page is setup correctly, sort out any crawling errors, and try to make checkout as simple as possible to potential buyers, group together related products, etc.
Once you get a structure you are happy with then look at investing any earnings into PPC. Just test it at first, and do not jump straight into it without doing some research. PPC could end up costing you more than you earn if not setup correctly.
Ecommerce stores can convert from anywhere between 1%-10% depending on the products you sell and yearly trends.
Play it safe. Ask your friends/family what they think, ask them their opinions and if they think the website is easy to use/navigate. That type of feedback is priceless
