Can you tell what caused it? It looks to have some rough edges like it rusted, and it also looks like there's some bodywork/bondo around it. Actually, is it really a hole? it almost looks like its a dent that someone patched over and then the patch fell apart (at least in the pic).
If you want to fix it I'd suggest that you start with sanding that whole area clean and find out exactly what you're dealing with. Start with some 40-80grit on a disc sander (in a drill, angle grinder...) and just roughly get down to the metal (don't cut into the metal at all if possible, that grit can grind through steel pretty fast, if you're worried about it and there's a bunch of body filler, you can rip that off fast without grinding into the base metal with a course wire brush), then show us what you end up with.
If that's pretty much all there is then you can fix it. Generally you'll want to cut the edges back till you have clean, full thickness (as in if rust or grinding thinned the metal) edges, and then In order of preference:
- cut a matching patch, shape it roughly like the area that you're filling and butt weld it in, grind the welds flush, a little hammer, dolly and body work and you'll never know it was there
- flange the hole (either by bending the metal by hand, hammer, a flanging tool or even welding another piece to back it up from the back), then finish the same as the first one. This one will require less skill and work to get it right, the patch doesn't have to match as well, and unless you're really good will be stronger, but could potentially be visible even with perfect body work under certain weather conditions at low view and lighting angles (read rarely but I've seen it)
- Flange like the second one and use panel bonding adhesive. this should be almost as good as the first 2, and actually the preferred method at some shops now. Fast and easy, and hard to irreparably screw up the panel (welding will cause some shrinking, and can sometimes pull the panel in directions that most can't fix)
- the above but using epoxy, glass reenforced bondo or fiberglass resin (with some strips of cloth between the pieces) to bond it together. I've gotten away with this before panel bonding adhesive got popular with dissimilar metals that you couldn't weld, but any decent stress on the area will eventually result in cracks
- finally, the worst that you can realistically get away with and make look good (as in hide the bodywork) is to lay up some fiberglass cloth. It can be made to (I think there are some videos how to do it using one of the repair kits on youtube), but honestly, it's more work than just putting metal in there. You basically chamber the edges of the metal and lay up the FG on some plastic with some resin, and then stick it to the repair area, shape it as close as you can get it and peal the plastic off...
With any repair, you want to do your best to get behind it with something waterproof (more pannel bonding adhesive, 'glass reenforced bondo...) and then even give it a coat of paint to make sure that no moisture can get in there (and you don't even have a gap in say a flange that moisture can sit in), or that's where rust will start.
FWIW, I doubt anyone, even someone that knows what they're doing, will get this done well in less than an hour or 2, so keep that in mind if you run across a good fender for a reasonable price (of course, if you don't have other dis-assembly that you have to do anyway, remember that it will probably take you a couple of hours, at least to swap fenders and it may take you more than that to get the front end sheetmetal well aligned)
