Navigate to sites like Toon University and Learn With Math Games. These services provide a range of free math teaching resources for first-graders, including interactive games and worksheets that you can print off. If you need additional activities and resources, some sites offer paid-for content that you can access on a subscription basis.
Invest in an interactive CD-ROM designed specifically to help your first-grader with his math. As well as helping your child with arithmetic, using CD-ROM resources will help your child get used to using a computer. If you can't find any at your local bookstore, you can order from websites like School Zone and Mathville.
If you're more of a traditionalist, you can find a range of math exercise books from bookstores and online retailers that have been written with first-graders in mind. Many include short activities that children can work through alone or fun ideas for learning math in a group.
Involve your first-graders in everyday tasks that will help them apply math to the real world. Encourage them to get a feel for money by discussing how much a general shopping item costs. Get them to work out how much change they would receive from a $5 bill if they purchased the item. You also can introduce a time-based activity. Tell your kids about how seconds and minutes fit into an hour and have them work with an analog clock to add and subtract time.