My car has been slow to crank the past few days. I have jumped it once. Now it tries to crank normally but will not. When I just tried to start it the horn started blowing and would not turn off for about 15 seconds?still will not crank HELP

Hook up the positive cable Red to the positive cable on the car to be jumped and the car used to jump the battery both cars off. Next on the car with the dead battery hook up the negative cable to the frame of the car if it is a liquid or lead acid battery, next hook up the negative cable to the car that will jump the other dead car. Warning an arc can cause an explosion if there is gas coming from either battery so when hooking up the negative cable be sure to the negative cable on the frame. Next start the car and rev it to about 800 RPMs hold it to 800 RPMS for about 5 minute's before try starting the other car, this will allow the other battery to charge up and will help start the car. If you have small jumper cables like #6 copper give it about ten minutes to charge before try starting or you could damage the alternator of the charging car. once the time has elapsed try starting the other car, push throttle down once while cranking it over and hold pedal half way till it starts, do not crank for more than 30 seconds if it does not start wait for the battery to charge up again to try starting. with low voltage condition if you crank the car and it slows down it can damage the computer by drawing to much amperage caused by low voltage. Remember not to crank for a long period of time as it can damge the starter and or the computer, if voltage drops below 10 volts it can damage the computer of the jumped car.