From what I have read, Jeep fenders can be notorious for rust. If it's not rusted through, that's good. I would just sand the rusted area down to bare metal until all the rust is gone (take care not to sand too big of an area, just around the rusted spot). Carefully feather (smooth) the sanded area with very fine grit sandpaper (maybe even wetting the sandpaper with water) until you can't feel where the paint and the bare metal meet. clean the area with a soft rag and some alcohol and let dry for awhile. Mask just around the area and spray a good quality auto primer over the area. Put on a few coats. Then lightly sand again with fine grit paper until you can't feel any edges between the primer and the unsanded paint. Then try to match the paint code to your vehicle, maybe with a touch up kit (basically a rattle can of your jeep color) either at a auto parts store or a dealer. Mask again, lightly apply 3 or more coats of paint over the primer and let dry. Following the directions on the paint can, wait the required Days and then use some slow cut rubbing compound over the repaired spot to blend it back with the original paint.
It may not look quite original but the other alternative is to take it to a body/paint shop. More moola needed for that. I'm sure a professional auto painter can give you better directions than I can but the above process is how I have always dealt with a rust issue. Best to get it before it rusts through.