Thank you for your question and welcome to Health Central's ADHD Community.
Usually, the first step is to request the school do an evaluation to determine if and how your child's ADHD is interfering with their ability to learn. When the evaluation is completed, you will be notified of a meeting time and will go over the evaluation with the school administration. At the meeting, if your child qualifies for accommodations, you can talk about what types of accommodations you feel would be best.
In the meantime, you may also want to set up a meeting with your child's teacher, ask about areas he is struggling (including behavior) and work together to find ways to help.
To read more about ADHD in school:
ADHD in Education
There is a lot of information on that page, try to read one or two of those articles you think are most relevant at first and then continue reading additional articles.