Here you are a very good link to a blog with a drawing and some exercises to practise: click here.
- THERE IS: singular, so you use A when it's necessary --- there's a car in the street but also you say there's (some) milk in the glass. You can say some to indicate the quantity
- THERE ARE: plural, so we say the number or a quantity --- there are some chairs or there are 3 children in the room.
To practise you have some exercises here:
- there is/there are: exercise 1 exercise 2 (a list of exercises)
- prepositions of place: exercise 1 (a list of exercises)