Learn Italian: Sdraiati sul pavimento immediatamente! - Lie down on the floor immediately!

This is an imperative (command) using the reflexive verb sdraiarsi (to lie down). When giving a command to tu (informal you), the reflexive pronoun ti attaches directly to the end of the conjugated verb (sdraia + ti).

Pavimento looks like the English word pavement. Do they mean the same thing?

No, pavimento is a false friend. It specifically means the indoor floor of a room or building. If you want to talk about the pavement or sidewalk outside, the Italian word is marciapiede.

Why is the word sul used here instead of just su il?

In Italian, you generally cannot leave these words separate. When the preposition su (on) is followed by the masculine singular article il (the), they must combine into the single word sul.

How is the adverb immediatamente formed?

The suffix -mente in Italian works just like -ly in English. To turn an adjective into an adverb, you take the feminine singular form of the adjective (in this case, immediata) and add -mente.