When you start a sentence with a phrase like this in sentence, you put a comma after the phrase.
Here are some examples:
After the cat ate dinner, she cleaned her face.
In the living room, the cat claimed the softest chair.
When the kitten realized he was big enough, he began jumping on the counter.
If the cats would stop scratching the furniture, I'd have a nicer-looking house.
Look back over those examples and take the commas out. You'll be able to see how the sentences run together without the commas and make more sense with them.
