Learn Russian: Прежде всего нужно узнать цену. - First of all one must find out the price.

It acts as a marker of priority, meaning 'first of all' or 'primarily.' While you are currently learning markers of addition, it is crucial to know how to sequence your points before adding to them. Literally, it translates to 'before everything,' formed by the preposition прежде (before) taking the genitive case of всё (everything), which is всего.

Why use узнать in this sentence instead of знать?

Знать is imperfective and means 'to know' as a continuous state. We use the perfective узнать here because the action focuses on the single, completed event of acquiring that knowledge, which translates perfectly as 'to find out'.

Why is there no subject like 'I' or 'we' before нужно?

This is a classic impersonal construction. Using нужно (it is necessary) with an infinitive and no subject makes it a general statement, meaning 'one must' or 'it is necessary.' To specify who needs to do this, you would add a noun or pronoun in the dative case, like нам (to us).

Why is the word for price (цена) in the accusative case (цену) here?

It is the direct object of the verb action. Even though the main clause is an impersonal нужно, the infinitive verb узнать still affects the noun exactly as it would in a regular sentence, requiring the accusative case.