I'm 14 and have suicidal thoughts?

I'm a 14 year old boy and I have depression but nobody knows. I go through waves of depression all the time and sometimes it gets to the point were I just want to grab the knife right in front of me and stab myself. Nobody knows that I'm like this although people know somethings wrong and I just make something up. My dad commited suicide when I was 4 and now I'm worried that I'm going to follow in his foot-steps. I'm scared to tell my mom because it might devastate her, as she had to talk to my dad for and hour face to face until he decided to take the gun and shoot himself right in front of her. I cry myself to sleep sometimes. The only reason I think I haven't killed myself yet is because I don't want my mom to go thru it again. Please Please Please Help!