First apply pressure to it, then elevate it above your heart. Relax, don't panic, slow your breathing down. Check it every few minutes to get an idea of how much blood is coming out.. Go to your sink & try to clean it up a bit around the area, any kind of antiseptic will work, & it'll sting. Return to constant pressure & elevation. If it appears to be slowing continue with the pressure & elevation. Hit it with more antiseptic, dry the area around the cut & apply a dressing & bandage. If after an hour or so the blood is still flowing, leave that bandage & dressing on & put a new 1 on top of the other, time for some stitches. You can do it yourself or go to the hospital
First apply pressure to the area: cover with a clean paper towel and squeeze the cut area firmly for at least 5 minutes. If this doesn't stop the bleeding, you need to go to an ER, ASAP. If it does stop the bleeding, you can apply neosporin and a bandaid--and change the bandage twice a day, very carefully (it's easy to reopen a deep cut). If you feel unsure about whether the bandage is enough, then the ER is in order.