Tilly, it's not difficult, but the one thing I'd say is get enough stout boxes and bubble wrap before you start selling - you must wrap very well, otherwise things get damaged in transit.
Postage isn't difficult. You weigh your item together with the box and wrapping, then use this chart (for UK and overseas sales) and then go to the price for the weight of your item - if it's borderline, go to the one above.
http://c435158.r58.cf...l_Mail_Our_Prices.pdf
I'd suggest that you try it with just a couple of items, to get you started, and see how you get on.