If you are worried that your friend might be suicidal, talk to them about it. Ask them if they are thinking about suicide. Be direct and use the word "suicide." Studies show that asking the question will not put the thought in their head, but will allow them to be more open about their feelings. It also helps you know how serious the situation might be.
Once you ask the question, give your friend time to answer. Don't rush them. Waiting for an answer may be nerve wracking, but don't fall into the temptation of nervous talking. Allowing your friend the time to respond gives them time to process their thoughts, and shows that you care about what they have to say.
Let your friend know that you are glad that they shared their thoughts and that you want to help. They may ask you to not tell anyone. If this happens, let them know that you care enough to make sure they get the help they need.
Even though this may be difficult information for you to hear, try to keep your emotions in check. Stay calm. Try to keep any feelings of shock, anger or surprise at bay. Be non-judgmental. This is not the time to debate if suicide is right or wrong or to give a lecture to your friend on the value of life.
Adults are better able to coordinate getting the help for your friend than you could do on your own. Think of someone that you trust and your friend will be comfortable talking to. This might be be your parents, their parents, a teacher, school counselor, coach, or maybe a youth leader at your church.
When a situation requires you to talk to an adult, be direct in letting them know that your friend is having suicidal thoughts and needs help to stay safe. If your friend has a plan for suicide or has attempted suicide, let the adult know this as well. Use the word suicide and be specific; don't just say things like "My friend is feeling down" or "My friend hurt herself." If you aren't clear, the adult might not understand the seriousness of the problem.
Although you should involve an adult right away to get your friend help, do not involve your whole circle of friends. Keep it private between you, your friend and the adult who is helping you. Talk to your own parent if you need support, but this is definitely not something that you should talk to other people about or post on social media.
Just because your friend is dealing with some heavy issues and getting some professional help, doesn't mean that you can't be friends. They are still the same person, and can still be your friend. Right now, they are focusing on getting help so they can feel better. When you are able to spend time together, find fun, positive activities that help distract them from their problems.
Even though you want to be a supportive friend, you also have to look out for yourself. You aren't a professional counselor so don't get bogged down with their problems. Be open and honest with your friend if things become too much for you to handle.