This is a good question.
It depends on if something can be counted or not.
For example:
“There is a lot of water in the ocean.”
(In this case, you use the singular “is”, because the water in the ocean cannot be counted.”
Or
“There are a lot of pastries on the table.”
(In this case you use the plural “are” because pastries can be counted, and there are more than one here.)
Good luck!