Dude, I'm sorry, but a monitor alone costs around $100. That leaves you with $300. Then you need an OS - that's another $90 unless you're going to run Linux Mint, or Linux Zorin or something like that.
For $200-300 the best advice I can give you is to buy a pre-built Dell. Sometimes I can find deals around $300 for a low-end PC. Not good for much gaming, but better than nothing. You really can't build anything for that low a price.
At a push we can get you something for around $400 - excluding the monitor, or maybe $500 with a monitor.
You can also watch Newegg and weekly they offer a bundle with some components for maybe $300. That would be your best option.