First off, make sure the bleeding has stopped. If the blood is gushing from an extremity, lift the leg above the chest (you'll want your dog to be lying on his back) and apply pressure below the wound-try to cut off the blood supply. Call the vet ASAP. If your dog has a cut that is slightly bleeding, simply place a clean dressing directly to the wound and apply pressure until the bleeding has stopped. If it's an older cut, wash the area with an antibacterial soap (Dial works well) and check for signs of infection- smell, greenish/yellow pus, redness, swelling. If the cut looks clean, apply an antibacterial ointment and wrap gauze around the area. Try to get an elastic bandage- if you use it, put a clean dressing around the wound first. Make sure you change the dressing twice a day. Good luck!
If its just a graze or scratch its best to keep it clean and dry, and preferably uncovered. If its a gaping wound that you have to hold closed, or pops open when the dog moves it may need stitches or gluing together, in which case you should see a vet.