Hi,
It is not that hard to use an exchange like MtGox, or BitFloor (or BitStamp if you are in Europe and prefer to pay with Euros).
Essentially, you register with the exchange. Then log in, click on "Add Funds" (the name varies by exchange). You probably want to make an ordinary bank transfer, with your budget the banking fee is probably insignificant.
At MtGox you can then choose to buy at market value, many other exchanges have a similar possibilities. Otherwise you just choose a reasonable price and place a bit for the amount of bitcoins you can get at that price. So what is the reasonable price? If you are doing a medium to long term investment, place the order at the current ask price and get the coins right away. If you are planning to day-trade, you can place your bid a tiny bit above the current bid price, and hope that somebody choose to sell to you soon. But the difference is going to be marginal compared to the long-term price drift.
Finally, I recommend using two-factor authentication if the exchange supports it, at least if you plan on leaving money on the exchange. Google Authenticator is really easy to use if you have a smartphone.
Happy bitcoining
PS. Bitfloor appears to be down today. Not a good sign. I have never used it, so I cannot recommend it, but many others do. MtGox is the largest exchange, but personally I am not a great fan of it, and I hear that sometimes bank transfers end up being converted from USD to Yen and then back to USD by the intermediate banks when they see that they are transferrring to Japan.
