If you take the caliper off the bike , remove the pads then gently squeeze the lever a little you can see if one or both pistons operate. If one stays still or doesn't move a great deal then it's likely it will need new pistons and seals. Best to pump the lever until one or both are out, you will lose fluid here so have a drip tray ready and rag as well. If you've a stuck piston remove it with compressed air. If it don't come out now buy a new caliper!
If you get the piston out hooray! Now we need some silicone lube on the new seals which having removed the old ones with a seal pick you can install in their grooves. Lube the back edge of the pistons and press into the calliper halves. Replace bolts and torque up, refit calliper, clean with Isopropyl alcohol (DOT fluids make great paint stripper) or similar stuff and get ready to bleed,
Okay, bleed kits. The Avid one is quite pricey, 25-35 pounds. Epic have a version @12.00. You'll need some DOT4 or 5.1 fluid. Have a good read of the bleed instructions before you start. The procedure is simple enough but seems complicated the first time you attempt it.
Having read all the above you may conclude that this time it might just be best to find a good bike mechanic and let them sort it out. If you don't feel competent to deal with hydraulics in this sort of depth it's what I would advise.
