Creating TRIGGER Need To Autoincrement PK

I am creating a trigger in SQL Server that will activate when my Athelete table has something "Inserted"

So far it works except for the fact that it won't insert a null into the Equipment_ID field. I need it to autoincrement, or do the last number + 1...

CREATE TRIGGER TRG_NEW_EQUIPMENT1
ON ATHLETE AFTER INSERT
AS
BEGIN
INSERT INTO Equipment Equipment_ID, Equipment_Model, Equipment_Year, Equipment_Brand, Equipment_Color, Equipment_Condition_Rating)
VALUES ('Big Spin','2016','K2','Blue','5')
END;
GO