Learn Portuguese (Portugal): Aqui há carros nas ruas. - There are cars in the streets here.

In Portuguese, the preposition em (in/on/at) must merge with the definite article (the) that follows it. Since ruas (streets) is feminine and plural, we use the article as. When em combines with as, it creates the mandatory contraction nas (in the).

How do I know whether to use no, na, nos, or nas here?

The contraction always has to match the gender and number of the noun that comes right after it. Because ruas (streets) is feminine and plural, you must use the feminine plural form nas. If it were a masculine plural noun, you would use nos.

Why do we use the singular verb há when talking about plural carros (cars)?

When the verb haver is used to mean "there is" or "there are" to show that something exists, it is impersonal and never changes to a plural form. You will always use the singular há whether you are talking about one car or many cars.

Could I put Aqui at the end of the sentence instead?

Yes, Portuguese word order is quite flexible. Saying Há carros nas ruas aqui is also perfectly correct. Putting Aqui (here) at the beginning of the sentence just gives it a little extra emphasis.