Kjøpesenteret gir en stor rabatt på datamaskiner i dag.

Gir is the present tense of å gi (to give). In Norwegian you literally “give” a discount. In English we usually translate this as “offer.” You could also use tilbyr (present of å tilby, meaning “to offer”)—for example:
Kjøpesenteret tilbyr en stor rabatt på datamaskiner i dag.
Both are correct, but gi rabatt is the more idiomatic choice in everyday Norwegian.

Kjøpesenter is a neuter noun. To make it definite singular, you add -et:
kjøpesenter + et = kjøpesenteret → the shopping center.
If you wanted the indefinite version (“a shopping center”), you’d say et kjøpesenter.

The noun rabatt belongs to the common gender (en-gender). Common-gender nouns take en as the indefinite article:
en rabatt = “a discount.”
Neuter nouns would take et (e.g. et hus = “a house”).

Adjectives agree with the gender and definiteness of the noun. For an indefinite common-gender noun you use the basic form of the adjective:
en stor rabatt.
If it were a neuter noun, you’d add -t: et stort hus. If it were definite, you’d add -e: den store rabatten.

In Norwegian you express what a discount applies to with rabatt på noe = discount on something.
på datamaskiner means “on computers.”

Time expressions like i dag (“today”) can appear at the beginning or the end of a sentence. Placing it at the end is very common in spoken Norwegian. If you want to emphasize today, you can also start with it:
I dag gir kjøpesenteret en stor rabatt på datamaskiner.

Datamaskiner is the plural indefinite of datamaskin. The sentence talks about computers in general, so we use the plural.
Singular indefinite: en datamaskin (“a computer”)
Singular definite: datamaskinen (“the computer”)

Yes. In ads or headlines you often drop en:
Vi gir rabatt på alt i dag!
Omitting the article makes it more general and punchy. Including en (gir en rabatt) specifies that it’s one big discount.