Suicide is the sin of “self-murder.” But it, nor any other sin, is not what decides whether or not we go to Heaven. That is decided ONLY by our relationship with Jesus Christ. If a person does not have a relationship with Jesus, he will go to hell perhaps sooner than if he had died naturally, but it is because he rejected Jesus, not because he committed suicide.
It is still very serious sin against God. It is saying to God that we know better than he does, and that we want the ultimate control over our own lives. Instead, God wants us to trust him to bring healing.