Use 'There is..." when the noun that follows is singular, and "There are..." when it is plural:
"There is/there's a pen and two pencils on the desk."
"There are two pencils and a pen on the desk."
So - "There's a dog and a cat in the garden."
We often use contractions in colloquial speech, such as 'there's' for 'there is'. This makes it quick and easy to say things like:
"Hey! There's a car in my parking spot."
Trouble is, there's no contraction for 'there are' : there're ????
So, even when the following noun is plural, colloquially we tend to say:
"There's a few people I'd like you to meet."