1. When do we use a lot of and when lots of?
These phrases are mainly used in informal English – lots of sounds a bit more informal than a lot of. Both forms are used in singular and in plural sentences.
It is not the phrase a lot of or lots of which determines singular or plural, but the noun of the sentence (here: water and computers).
i. Informal English
A lot of water is wasted.
Lots of water is wasted.
A lot of computers are needed at schools.
Lots of computers are needed at schools.
ii. Formal English
In formal English we use plenty of or much and many instead of a lot of/lots of.
Plenty of water is wasted.
Much water is wasted.
Plenty of computers are needed at schools.
Many computers are needed at schools.
2. When do we use much and when many?
much: uncountable nouns (milk, marmalade, money, time etc.)
many: countable nouns (bottles of milk, jars of marmalade, dollars, minutes etc.)
How much money have you got?
How many dollars have you got?
In informal English these questions are often answered with a lot of, lots of. There is no much difference between the two phrases.
3. When do we use a little/little and when a few/few?
a little: non countable nouns (milk, marmalade, money, time etc.)
a few: countable nouns (bottles of milk, jars of marmalade, dollars, minutes etc.)
He has a little money left.
He has a few dollars left.
We use few and little without the article a to point out a more negative meaning.
A few students of our school know this. (There are some students who know it.)
Few students know this. (It is almost unknown.)