If you take your bike riding seriously you should do yourself a favor and buy top quality cycling clothing. Not only will you look great when riding but your comfort level will be greatly enhanced which will in turn, positively affect your overall performance.
You should own some basic, high quality clothing such as a good long sleeve cycling jersey for cool weather riding and a regular, short sleeved or sleeveless jersey to wear when the weather is warm. It is never a good idea to wear a regular cotton t-shirt because the shirt will become damp when you sweat and will cling to your skin, making you feel very uncomfortable quickly. The $100 or so that you invest in two high quality jerseys will definitely be money well spent as riding will be much more pleasurable.
You will also need some good shorts. Consider looking at Pearl Izumi shorts as these are some of the best on the market. These shorts are ergonomically designed to deliver comfort, durability and a great fit. The fabric used keeps moisture at bay and these shorts feature padding which absorbs road shock.
You can find the best cycling clothing on the internet. There are many online retailers specializing in nothing but cycling clothing so take the time to do some comparison shopping to ensure that you get a good deal.