Thanks for your question.
Suicide is self-murder and it is most definitely sinful. By committing suicide, a person is saying, "My problems are too big for God to help me solve." This is a lack of faith. Whatever denies faith is sin. See Romans 14:23 "...for whatever is not from faith is sin."
There may be some exceptions to the rule that suicide is sin. Some people are beset by a medical/mental condition which leads them to take their own lives. They may not be responsible for their actions. We know that God judges righteously and will not punish a person for doing something over which he or she has no control. Such a person would do well to take advantage of the wonderful advances in medical and psychological science available to us today to help in dealing with such conditions. These advances are blessings from God and we would be foolish not to make use of them when the situation calls for such intervention.
If you want to come to grips with what the Bible really teaches, why not take the free online Bible study course available on our website, www.thisisyourbible.com? This course will give you a background in the major themes of the Bible from Genesis to Revelation. As with everything we offer, there is no cost to you other than your time and effort. You will also have a personal tutor to whom you may pose questions either from the course itself or those questions which come to you as you read the Bible.