You can introduce the idea of numbers when your child is as young as 12 months by counting small sets of items — "How many buttons? One, two!" — and singing songs and rhymes that include counting, such as "One, two, buckle my shoe," "Five little pumpkins sitting on a fence," and so on. When your child is 2 years old, she may learn to count up to 10 by rote, though she won't really understand the concept of counting objects yet, and may skip around in her counting — "One, two, five, six..." Don't worry about her mistakes in counting — the fact that she's reciting numbers means she's learning the correct names. Next she'll learn to point to objects and label them with numbers (even if they're not the correct ones). Take advantage of everyday opportunities to count together; setting the table is a good example. "One napkin for mommy, one napkin for daddy, one for you! One, two, three napkins!" At first your child may say there are three napkins no matter how many you've actually laid out, but at some point she'll start to understand that the word "three" stands for the number of napkins.
As she gets a little older — between 3 and 4 — your child will start to understand the idea that adding more objects increases the number counted (whereas taking away objects decreases the amount). So when grandma comes to visit, there's another napkin on the table, bringing the count to four. Another way to reinforce this concept is to count everyday objects in your child's environment — the number of apple slices on her plate, for example — and note what happens when objects are subtracted (eaten) or added. Between 3 and 4 years of age, she'll also be more adept at counting small sets of objects — "two oranges, four straws" and so on. Most children are not able to identify numerals or write them, though, until they're 4 or 5.