There are many websites and forums out there designed to help, maybe- if possible- get them to see a therapist or someone professional if you think that suicide is something that they're capable of doing, and think it would help.
If they don't want this the most you can do is be there for them, look after them etc. Listen to them, find out why they want to kill themselves do anything within your power to drag them away from that possibility- watch their favourite movies/ tv series with them, listen to their favourite (HAPPY) music with them (music is meant to have a direct effect on your mood, therefore happy=good).
If there is something fun they've been interested in for a while, do that.
Get them into a new hobby that they might enjoy- instruments? Singing? Games? Sports? Cooking? Baking?
but make sure that it is healthy.
If it is not within your power to dedicate time because you are very busy, just dedicate as much time as you can to them, encourage any other friends of theirs to do the same (although maybe in a maybe in a more subtle way, if it's only you that they've confessed their feelings to).
If you're younger, you may want to consider telling a trusted adult- a parent or guardian, school councillors (I have no experience with these, and don't really know much about them personally, so maybe not), or a teacher.
I sincerely hope that this helped and that happier days come to your friend soon.
Best wishes and best of luck.
