Children need to hear counting often to commit it to memory and be able to parrot it back. They need a lot of practice with one-to-one correspondence to understand that the numbers we say are attached to the objects we are counting. (Ever see a child speed through the numbers and point to only one or two items, or double count lots of items?)
Count everything you can with your child and count slowly, emphasizing each item. "One apple, two apples, three, four, five apples!"
One way you can make counting a little different and fun is by putting ant stickers inside baby food jars. Jar one had one ant sticker, jar two had 2, and so on up to 6. (I couldn't get more than 6 stickers to fit in the jar.) The child has to feed each ant a marble. The sound of the marble hitting the jar was delightful and made my 22 month old want to keep feeding the ants over and over. (You can use any other sticker and any other type of manipulative as a counter. The idea is to count out as many counters as there are stickers.)
Now feed the ants!
There's even a picture book called One Hundred Hungry Ants by Elinor J. Pinczes that would go well with this activity.