We'll need some more information about the light before we can answer your questions.
1. Describe the bulb. Is it a long tube with pin connectors at both ends? That would be a fluorescent bulb. If it's got a screw base at one end, it's probably an incandescent bulb. There are other options, but it's rare to see them included with a tank.
2. I have a hunch that you have a fluorescent fixture, in which case you have a little less than 0.5 wpg
on a fairly deep tank. Definitely not a good combination for plants. If you have an incandescent fixture, it's even worse. You might be able to grow java ferns or java moss. Floating plants may also work as it eliminates the depth factor.
3. A fluorescent fixture can be upgraded, but it's probably not worth the effort. You'd have to get a retrofit kit and hack up the fixture. An incandescent fixture can be upgraded with CFL bulbs. If you can find them, U-shaped bulbs are better than the spiral-type, but both work.
If you want to grow plants, I suspect you'd be best off upgrading to a T5 or T5HO fixture or at least making a DIY fixture using several CFLs. I'm guessing your tank is 36" long. There are a lot of fluorescent options out there.