Know your metals!
Brass - Brass is a combination of copper and zinc. If you are sensitive to copper your skin may turn green with contact. Brass will always tarnish, even if its coated with a protectant. It can be polished to look like gold, but the patina is what makes it so beautiful right?
Sterling Silver - Sterling silver is about 90% silver and 10% copper or zinc. Like brass, sterling can tarnish too.
Gold Plated - Plating is a process where a thin layer of gold covers a base metal which is usually silver or copper. The plating can wear off due to wear from rubbing, perfumes, or your skin's own pH. If plated jewelry looks like its tarnishing, that is actually the base metal showing from the gold being worn off. No worries though, you can go to a jeweler to replate the piece with more gold.
Gold Filled - Gold filled is a thicker layer of gold bonded to a thin piece of base metal. Unlike plating, gold filled is not likely to wear off. Gold filled jewelry makes a great alternative to solid gold because it not as expensive.
Solid Gold - Solid gold is solid gold. No other metals are added so this metal is great for people with sensitivities.
Polishing and General Care
Tarnish occurs naturally but its always good to try to avoid it by storing it in a cloth pouch or in a jewelry box. Store your jewelry in dry areas, do not leave your pieces to sit in your bathroom. If you want to make your pieces nice and shiny again then you have to clean them by polishing it. To clean your jewelry I recommend using a polishing cloth. Polishing cloths like this are my favorite because they last a long time and can be safely used on all types of metals. You can also use liquid chemical tarnish remover but make sure its safe to use on the metals you have. (Sometimes they are specific to only one type.) There are also natural recipes to clean jewelry using baking soda and lemon juice but I find that they do not work as well. Remember to avoid polishing plated pieces because you'll polish off the gold.
No matter what your jewelry is made of, you should never go swimming or shower with your jewelry. Avoid wearing your jewelry when working out and playing sports. Do not bend your jewelry to see how strong it is or to make it fit. If you have to bend something to make it fit then it doesn't fit. Taking your jewelry off at night may also add longevity to your pieces. Treat your jewelry with care and you will enjoy them for a long time.