Learn Italian: Forza squadra, segniamo nella rete! - Come on team, let's score in the net!

Segniamo is the noi (we) form of the imperative, which we use to give commands or make group suggestions. For regular verbs like segnare, this form is exactly the same as the normal present tense. The context and the exclamation mark tell us it means "let's score!" here instead of just stating a fact like "we score."

Does forza always mean "come on"? I thought it meant "strength".

You are exactly right that forza literally means "strength" or "force." But when used as an exclamation like it is in this sentence, it is a very common way to cheer someone on, meaning "come on!", "let's go!", or "you can do it!".

If I wanted to tell just one player to score, how would the verb change?

To command one person (the informal tu form), you would use segna. For regular -are verbs like segnare, the tu command form ends in -a. You would say: Forza, segna nella rete!

Why do we use nella instead of just in before rete?