I cut my forearm and it's open and red all round and really tender what should i do ? self harm cut

See a doctor : Without knowing when this happened or its course thus far or how deep it is, one can't say what is needed. Therefore you need to see a doctor in case some additional treatment is needed, like repair or antibiotics.

Medical Help: If you have gapping skin, increased redness, swelling and tenderness - then you need medical attention now. Don't know when you cut but you might need sutures or possibly treatment for infection & tetanus shot.