Niche work might be better than being a more general web developer. There is less competition and you can generally charge better rates for the niche stuff. So you might start out looking at what work is out there for the FileMaker side (I know nothing about that.)

Also, managing projects isn't a bad way to go. Making sales is probably the most important part of web development. If you can make the sales then you could start building a web development team (probably a remote team) and have them do the work while you manage the projects. This is risky because devs can flake out on your or you could lose money by underestimating jobs but if you get it right then it could be a better money maker than working solo.

If none of the above will work for you now, then you might be best off starting out building sites in something like Wordpress, Drupal, ExpressionEngine, etc. The reason is that there is a huge market for working existing designs (I'm assuming you are a developer as opposed to a designer)into a CMS so the client can easily update the site. A CMS like Wordpress is pretty easy to get started with, doesn't require a lot of technical skill and often you won't need to do anything crazy with PHP (ExpressionEngine requires little to no PHP knowledge.)

This will get you started. If you feel that you want to go more the route of custom applications rather than working with content management systems then you could start branching out as you get more comfortable with your dev skills. At this point, go ahead and start researching different scripting languages and the tools / libraries available for each to decide which is the best fit for you. Again, keep your market in mind for it. PHP is a great money maker because of the market size, but you also have a lot more low cost competition. The more niche you go, the lower the competition and the better the rates, but you might also have a harder time finding work.

There have been a lot of other posts on hacker news for how to get started. If you already have a great source of work then you are already ahead of the game. If not, then you need to start making a name for yourself. You could hit the pavement and look for local clients but you also need to be able to bring in clients from elsewhere. Use your knowledge to help people where paying clients hang out (Wordpress help forums for example.) Build Wordpress add-ons and give them away (but selling add-ons could also be a good money maker.) Setup Twitter for answering questions and finding people who are looking for paid help. Build your freelance site and use that site for support for your add-ons and posting tutorial

I mentioned Wordpress as an example, but you could substitute that with any CMS. Potential goldmines are new up and coming systems which have exploding numbers of new users but have a relatively low developer base. If developers are relatively hard to find, but the demand is high, then you have as much work as you like. I can't think of any examples off-hand, so you would have to do a bit of searching. Also, paid systems are great because the clients interested in those systems are already willing to dish out some cash and lower paid competition generally sticks with free systems.

As for advancing technology, it doesn't change as fast as you might think. The bleeding edge moves fast and you have to keep training yourself, but the edge must still keep the caboose in sight.

Bonus: Learn Javascript as well as one of the more popular frameworks such as Jquery. It doesn't matter which server side scripting language you learn, you will be using JS.