You should start with mineral spirits (paint thinner). Road tar is a hydrocarbon as is mineral spirits. Use gloves and put some mineral spirits on a rag and work on small areas at a time. Do it outside as well. For gloves go to a hardware store and get nitrile gloves (should be green or blue). Depending on how much tar you got on the bike, it will be messy so don't wear your "Sunday Go to Meetin'" clothes.

If mineral spirits doesn't work, try kerosene or diesel fuel but be extra careful with both. You might look at an automotive store for tar remover but be careful with it since I don't know what it might do to bicycle paint and parts. Don't use gasoline because the flammability is just too high for it to be safe. Alcohol, acetone, methyl ethyl ketone, lemon juice or a whole host of water based remedies won't work because tar is tough stuff that is not soluble in water based solvents.