I would never suggest that someone not use Umbraco as I personally think it is awesome, but I would ask why you chose Umbraco and not a 'dedicated' .Net ecommerce platform?
There is a package available for Umbraco called uCommerce which has fantastic reviews, and there is a free version although it has certain key restrictions, like marketing aspects and the multi-language/multi-currency version has an annual charge. However if you are new to Umbraco but not to .Net it may be better looking at something like nopCommerce which is free and open source.
Umbraco is a fantastic CMS and allows for the most flexible of site creation - but that is what it is, a site creation tool. nopCommerce is an out of the box ecommerce application that provides you a pre-built shopping website and administration area.
The only downside to nopCommerce is that it's CMS features aren't brilliant. By that I mean although you can create non-shopping pages, you can't create bespoke page/content structures in the same way you can in Umbraco.
Either way, before developing in anger I would suggest checking what your requirements and priorities actually are. Do you want a CMS with ecommerce features, or a ecommerce platform with CMS features?
Update:
Since writing this I have used an Umbraco package called uWebshop which is full-featured but slow, buggy and difficult to extend as the documentation as lacking. It is open source so I was able to delve and locate various extension points but it had too many bugs and the project was ultimately shelved.
There is another Umbraco package available called Merchello, again open source, which I am hearing very good things about and the documentation looks quite useful.