It depends on the problem, age of your child and type of school.
For example, if your dc is at a comprehensive school, chances are they will have some sort of pastoral support team you could approach. If its more a learning problem, there will be heads of years etc.
If your dc is still at primary school, generally their class teacher is a good place to start or again, head of year/key stage.
I usually find the most productive approach is to go direct (by email, phone message, or letter) addressed specifically to a named member of staff, who I know is either responsible for that particular area, or I think may be responsible - but will pass it on if not.
If a primary school, I'd contact the class teacher (via school office or email if you prefer) and arrange a meeting with the teacher. Don't go over the teacher's head without giving them an opportunity to sort things out first.
If it's a bullying/welfare type issue, then some schools have a home school liaison or learning mentor who's job is to help and you may be pointed in that direction by the teacher.