Well, you can always learn the theory, but practical experience is something you have to, well, experience!
I'd say dealing with suppliers, procurement and stock control are the hardest areas. Trying to find out where to buy your stock, how to "negotiate" volume discounts, retro sales rebates, advertising/marketing contributions, credit terms, sale or return, etc etc. You can only get that with experience - no one can teach you how to get the best deals, what kind of deals your suppliers give to others etc. Basically you need to learn how to haggle and that comes from experience, making relationships with suppliers/factories etc.
Try to avoid using wholesalers - all your competitors would be buying from the same places and wholesalers sometimes make higher margins than retailers (especially in food!). Try to buy from as close to the manufacturer as possible, i.e. direct from the factory, or at least from the importer/distributor, or if hand-made/crafty items, straight from the maker. Do your utmost to by-pass wholesalers!
Try to source unusual/unique items, maybe even by working with factories/manufacturers for them to make stuff especially for you, and you alone, that no other shops (or online retailer) are going to undercut you on price!
Re stock control, be very wary of buying too much stock. I presume you don't want to do a "pile it high, sell it cheap" kind of race to the bottom shop, so be particular as to your product range, keep orders/stock as low as possible, but at the same time, try to give a good choice/range to your customers. It's a balancing act really. Having too much money tied up in stock will jeopardise your business as you'd have to sell it off at a loss if you hit cash flow problems. Also, obviously, if your products have "sell by" dates, i.e. food, medicines or other perishables, even more important not to have too much stock! Sometimes, you really can have too much stock - if your shop is cluttered, it may put off customers if they get confused with too much choice or a confused range of goods.
Unfortunately, none of the above can be learned in theory! Well you can get the idea of the theory, but real life experience is the only way really to do it right!