The most important thing is to choose pieces that you like. If you don't like something, whether it is a particular metal or style, there is no reason to add it to your collection, even if everyone tells you that it is something you should have. I have a strong preference towards silver, so the bar is set much higher for gold pieces and I rarely buy them.
Make sure that you have pieces for a variety of dress codes. I prefer simple styles with a pop of sparkle for semi-formal occasions and brighter colors for casual wear. When I first started my jewelry collection, I only had pieces for dressy events because those were the outfits that most needed to be accessorized. I wore several weird outfits consisting of basic t-shirts and glitzy jewelry before I decided I needed to pick up fun costume pieces that could be easily worn during the day.
Set a budget and stick to it. There are great options at every price point so don't be discouraged if you have to expand your search beyond your favorite stores. Although I can find great deals on clothes at Ann Taylor and Banana Republic, their jewelry generally feels overpriced for the quality. For inexpensive pieces, I like Charming Charlie (though you have to sift through a lot of not-so-great pieces to find the gems) and for something a little bit nicer, try your local boutique for a unique option.