Wilton has a tip just for that job. I'm also pretty sure that other company's have the same type thing too. Here's a link to the Wilton website showing you what it looks like:
You can also use a tip 21 in a pinch. Just shove it into your cupcake all the way up to your coupler (if you are using one) and squeeze. How much you put in is your preference, but if you squeeze to hard or for too long you can blow out your cupcake.
Hope this helps,