hi,
I'm new here as well and I don't have direct experience selling a clothing line but I have been researching it for my own design company and have some experience with vendors and outsourcing shirts,labels, and printing for our clients.
1) there's an old saying you can have it fast, cheap, or good pick any two. i first saw this on some IT guys wall a long time ago and it is a good standard for any service. here is a more detailed description if you are interested
Fast, Good or Cheap. Pick two. I would try to find a single vendor that will use the brand and quality of t-shirts you want and have them screen printed for the best product. negotiate with them to get the price down if you can give them a good initial order or promise some consistent buying every month. Before you try to pinch pennies make sure to research the lowest wholesale prices for the t-shirts and have an idea how much it actually costs to do a multi color screened shirt.
if you only have a few shirts and will sell direct find a good template and setup you site and use paypal to handle payments. I do a lot of e-commerce and paypal is easy to setup and has a relatively low fee base and has good name recognition. When you start making money move on to a real merchant account from authorize.net or similar and setup a real shopping cart site to get the lowest possible per transaction fee and have more control over your sales and refunds without the paypal babysitting.
2) unless you have some real talent in design I would just try to make a good pencil / paper sketch and find a good designer who will work with you to make it a finished design.
3) yes and my company has designers our fee is $90hr or a flat fee based on project. your best bet is to use a crowd sourcing site for graphic design that has a bunch of people trying to "win" your bid. if it works out on the first job try to maintain the relationship and see if you get a better price for multiple jobs. there are a lot of "crowd source design contests" sites, just do a search.
hope this was somewhat useful info. good luck.