Every once in a while a friend says "Hey Mike, you're into bikes: What do i need to know to buy a road bike?" I'm sure there are plenty of other good guides on the internet, but because you asked, here's my two cents:
1) Fit - Having a bike that doesn't fit you right is the most guaranteed way you won't enjoy riding your bike. Plus, it can cause some serious injuries if you ride hard and long on it. Head over to this really cool fit calculater and get some idea for what frame size you need. Note: Different brands have diffrent geometries (That is to say, different shapes, angles, and lengths). For road bikes I wouldn't say one brand is above the rest, rather that they're all offering something different as far as materials and geometry. If you're new to cycling, brand and geometry probably don't matter as long as the frame feels good. If you're abnormally long or short in some area (legs, torso, arms) than it is possible that one brand may feel much much better than others. So make sure you give your bike a test ride.
2) Components - Ok, so there are two main brands out there: Shamano and SRAM. Each brand has different levels of performance, from the stuff the pros are riding in the grand tours to entry level stuff. Don't be oversold, especially if you're buying a new bike and are new to cycling. If you're not racing, you don't need the highest level of components. Even if you ARE racing, you probably don't need these. Each Brand's line is as follows:
|This is the level I ride||Rival||105|
|Honestly, fine for any beginner||Apex||Tiagra|
You may notice that I left Shamino's "Sora" group off the list. That's because while I did say don't be oversold, I wouldn't bother buying any with these components. In my opinion, if you have a choice, SRAM components at a similar "performance level" are always better than the Shimano brand. So if you're choosing between two equals and one has SRAM and the other Shimano, go with the SRAM.
Now that you know your size, and arn't clueless to what the components mean, you're ready to shop.
3) Look at used bikes on Craigslist - If you're looking for a used bike, start by scanning your local craigslist. I find the best way to search is to type your size (Say "54 cm" or "54cm") into the search bar under the "bicycle" section of craigslist. If you don't like what you see, or want to keep looking to see if anything new pops up: set up an RSS feed on craigslist with your size in the bicycles category.
Also, keep your ears open for bike swaps. In the Boulder/Denver area, VeloSwap is a pretty amazing sale for used bikes.
When looking at used bikes, be sure to check for scratches on the shifters and the derailleur, as these indicate the bike may have been in a crash. If your bike is carbon, make sure there's no scratches that go through the clear-coat and into the carbon, or the bike will be structurally unsound. If you're looking at steel, check for rust. If you're looking at aluminum, make sure there are no big dings or scratches in the frame.
4) Go ride some bikes - if you're buying new, look for sales and shops that offer good service. Bike's arn't maintenance free, and if you're not doing maintenance on your own: getting regular tune ups can add up. Try to find a shop that offers some sort of service package with a new bike. Just remember: when talking to the salesman don't be oversold.
5) Accessorize - Make sure you leave room in your budget for a few other add-ons that can add up. You'd be an idiot not to wear a helmet. Bike shoes and pedals are a must. I ride a generic look keo knockoff, but i've also heard great things about speedplay pedals. Bike shorts? Goofy looking: Yes. Necessary: yes. if you opt out, you're looking for chafeage and a sore butt. Finally, make sure you get a pouch to carry tire levers, an extra tube. Have some sort of pump or CO2 cartridge system, and a way to carry water. You don't need to walk out of the shop looking like you're on the Garmin team... but you don't want to be riding with a backpack and jean shorts either.
Happy Riding!! Oh, and just a quick note. I have mixed feelings about buying online. For one thing, you can't see how the bike feels, which is important, but there are some great sites out there with amazing deals. So maybe check out Performance Bike, Jenson USA, and bikes direct to see some cheap online options. That said, i've owned 3 road bikes, one from a bike swap, one from a bike shop, and one from online. I love the deal i got on the online one, and never really used the bike shop's service package because I moved and started doing my own maintenance. One cool thing about Performance is they offer 10% on purchases back in "points" which can be used to buy other things like bike parts, clothes, and accessories.