The good news is there's nothing mysterious about hydraulic brakes, the bad news is you might need to spend some money. Don't know about you but for me a front brake that works is a very high priority. Pretty much like refurbishing anything, get the brake off the bike, dismantle the brake line, master cylinder and caliper. Often the piston is stuck inside the caliper - penetrating oil and wait and if that don't work move on to further measures. If it's the original rubber brake line, I'd recommend upgrade to a braided steel line, one piece with no union half-way along. Clean the master cylinder, there are kits to replace the piston and spring but it's worth thinking about investing in a new m/c with a smaller piston - I did this and well worth it for a much better 'feel' at the brake.
Post photos and questions as you go . . .
Post photos and questions as you go . . .