Simple answer to a 0303 code?

I would start with the spark plugs.

There's a bunch of stuff you can do for little or no cost. Pull plugs and look for fouling. You can swap them around and see is problem follows. A tune up may be in order; plugs, plug wires, cap/rotor if needed. You can often borrow tools from most auto parts stores through their loan program. As long as you have plugs out, you can use a compression tester to look for loss of compression (indicating bad valve seating or head gasket issues). If you have a multimeter you can test coil packs. You can borrow a noid light to check injector pulse. You can also swap around injectors to see if problem follows. (not all vehicles make access to injectors easy tho.) Sometimes a sticking EGR can show up as a misfire on a specific cylinder, you can borrow an EGR vacuum checker as well. (axleaddict.com has a good 'how to test an EGR' page.) Just a few suggestions to get you started, and again, most parts store loan tools (compression tester, EGR tester, multimeters, noid light, and so on.)