Mimi ninafanya ununuzi mtandaoni kila jioni.

Mimi is the pronoun “I.” In Swahili, the subject prefix on the verb (here ni-) already indicates “I,” so including Mimi is optional. It adds emphasis, contrast, or clarity. You can omit it:
Ninafanya ununuzi mtandaoni kila jioni.
still means “I shop online every evening.”

ninafanya breaks down into three parts:
• ni- (first person singular subject prefix, “I”)
• -na- (present tense/habitual marker)
• fanya (verb root “do/make”)
Put together, ninafanya means “I do” or “I am doing.”

kufanya ununuzi means “to do shopping” (literally “to do a purchase”). You use the noun ununuzi (“purchase/shopping”) with kufanya (“to do”) when you speak of the general activity of shopping. If you want to emphasize the actual act of buying, you can say:
Ninanunua mtandaoni kila jioni.
(“I buy online every evening.”)

Yes. Swahili allows flexible word order for adverbials. You can say:
Kila jioni ninafanya ununuzi mtandaoni.
It still means “I shop online every evening,” with the time expression up front.

Both -na- and hu- mark present habitual tense.
• -na- is extremely common in everyday spoken Swahili.
• hu- is more formal or literary.
So you could also say:
Mimi hufanya ununuzi mtandaoni kila jioni.
and it would convey the same habitual meaning.

To negate the present tense, replace ni- + -na- with the negative prefix si- and change the verb’s final -a to -i.
Using kununua (“to buy”), you get:
Mimi sinunui mtandaoni kila jioni.
This literally means “I do not buy online every evening.”