When I first considered starting an online business many years ago I wanted to sell something that I already knew allot about. Being a musician my entire life it only made sense to focus on the instruments that I played. As a drummer my knowledge and experience was vast so I began my search for products with the cymbal industry and then moved to acoustic and then electronic drums. Eventually I included guitars and pro audio to the mix as well. The first place to begin is with a website. You can use hosting companies such as Homestead or Go Daddy which allow you to build your own site using one of their templates or have a professional web designer build it for you. Either way build the site to look good and be easy and functional for the visitor. Next add your products. You will need to contact the companies individually and request to be a dealer or use a distributor that offers many product lines. You will have to submit an dealer application with each. Once you've been accepted as a dealer you can decide whether to stock merchandise or have your suppliers drop ship your orders to you customers. Next set up a merchant account and shopping cart for your site to allow customers to check out on your site. You can start with a basic Paypal shopping cart and then move to a traditional merchant account like Authorize.net. Next promote your site and build search rankings to improve traffic. This can be done by hiring a professional SEO person to optimize your site or you can do it yourself my building links to your site and choosing good keywords. You can also use Google Adwords to move to the first page fast but keep in mind if you don't convert those visits to sales you may end up loosing money this way.

That's the basics of starting a music retail site on-line.

Good luck.