You may be getting divorced from your partner, but you're not getting divorced from your kids. They'll continue to need you, probably even more, during this challenging time. Teens are old enough to understand what's going on when their parents split up, but that doesn't mean they need less help in dealing with it. They'll probably run through a whole gamut of emotions, ranging from guilt to anger to pretending they don't care. Let them know that you empathize with their feelings and particularly stress that the child didn't cause the divorce -- it's about the relationship between the parents and not about either parent's relationship with the child.
A divorce will probably mean a lot of changes in your teen's life: There will need to be a schedule of time spent with each parent; your child may have to move house or change schools; one or both parents may have a new partner (who may also have children); there may be financial adjustments too. It's important to talk through these changes with your teen, but don't expect him to become your support -- he's still a child. Let him know that most of the decisions are not his, but do let him voice his opinions and have a say when appropriate.
If at all possible, try to maintain civil relations with your ex, at least while your kids are around. It doesn't help anyone, least of all your teenaged children, if they hear you denigrating their other parent, or if you expect them to take sides. Ideally, there should be a level of friendly cooperation between the divorced parents -- be a positive role model for your children. While family events will usually become one-parent affairs, for some important events in your child's life, such as graduation, major sporting events or recitals, she may really want both parents there, and it's good if you can manage that. Stay in regular (but not suffocating) contact when you can't physically be with your teen.