You should never start a new business venture without first researching the field, and in researching the field you need to research your competition. You may find that you will have a lot of competition, which may dissuade you from the field. You may find that you have a competitor that is so impressive that you will likely never offer a better product, or in this case a website, than them. You may find that you won't have a lot of competition, which should bolster your confidence for the future. The point is that no matter what you find out the research that you do before you break ground is essential.
The investigating you do should be detailed in ever manner. Your competitors have been through everything that you will go through so learning from their mistakes can ensure your success in the future. You'd be amazed what you can find out if you just ask the right questions. So email your competitors as a fan or user of their website, maybe as student or journalist. If they have a feature that you really like but don't know how to do ask them, say you're an aspiring web designer and were impressed. Try to find out how much money they're making, find out who is serving their advertising or if they sell it themselves. If they sell it themselves then you can estimate how much they make by looking at their media kit. If they go through an agency you can usually tell which one by looking at the source code. Remember, the agency and the rates you find, when you look for advertising after your site is done this information will come in useful. Another clue to the profitability of the site is the ownership. You can usually tell if the site is owned by a business or a single person by looking at the WHOIS information for their domain. If it's owned by a business then you can assume that they have more funding than you, but in contrast they also have more overhead and because of that overhead they probably aren't quick to adapt to new business environments. Also if they're a business then they may be susceptible to a downturn in the economic climate, in other words there is a higher chance that they'll go belly up. If the site is owned by a single person then there is little to no chance the site will go belly up, with little or no overhead being profitable is easy and so the owner will likely keep his website for a long time. Of course a single owner doesn't have the funding of a large corporation so any costly expenses will probably be done without.
Another thing to check up on is your competition's search engine rankings. You need to know what you're up against. Once you learn about how search engines and directories work you will be able to analyze whether or not there is real competition for your subject matter. Even big companies often overlook good search engine and directory optimization, or they focus too much on branding so you'll be able to beat them in the engines even if they have ads on TV. For more information on sizing up search engine competition check out our article on choosing effective keywords.
If you are intimidated by your competition, do not despair. Instead try narrowing your topic some. For example instead of web development in general, instead focus on scripting, or go a step further and just focus on server side scripting, or go a step further and just focus on JSP. You may not be able to compete with all the web development sites out there, but you might be able to compete with the JSP focused sites.