A lot of and lots of have similar meanings to much and many. They are mainly used instead of much and many in affirmative sentences in informal situations. However, they can also be used in questions:
There’s a lot of pie left. Help yourself.
Did you buy many/lots of souvenirs in Rome?
Tons of is even more informal than lot/lots:
“Do we have any fruit left?” “Yes, there are tons of oranges and some apples and pears, too.”
In a more formal style we use a great deal (of) before singular uncountable nouns and a large number (of) before plural countable nouns:
He inherited a great deal of money from his uncle.
A large number of complaints have been lodged against the company.
Several means more than one, but fewer than many. It’s used with plural countable nouns:
Several people have lodged a complaint against the company’s executives.
It is the subject that determines whether the following verb is singular or plural. So when a lot of/lots of are used before a singular subject, the verb is singular and when they are used before a plural subject the verb is plural.
A lot of/Lots of time is required to study for a doctorate.
A lot of/Lots of people are scared of heights.
Think about your possessions. Write some sentences in your notebook describing the ones you have a lot/lots/tons of.