Well, you have several options.
1. By a tuner(amp)/speakers/cables seperately (probably not the best option on a $300 budget, especially if you want surround and a subwoofer)
2. Buy a "Home Theater System" which usually comes with a DVD Player/amplifier and your speakers/sub
3. Buy a boom-box type unit with stereo input (some of these have surprising sound quality)
It really depends on what you want to do with it. How many devices do you want to input (do you have a bluray/360/PS3 etc.)? How big is the room?
I would reccomend looking at combo-style home theater systems. There are some in your price range, though they can get pretty pricey.
However, if you are looking for a quality system to last a long time and expand your home theater around it, by a higher-quality combo-system or buy the pieces individually. One last tip is to try out some different options at a store first, if you really want to purchase online (just remember it won't sound exactly the same at home).