There are several ways to express the meaning behind “if” in Japanese. なら is one of them. It means “if the preceding phrase is true, then…”
It may be best to memorize example sentences.
-i Adjectives: Adjective + なら [You can also use の: -i Adj + の + なら]
takai nara, kawanai.
If it is expensive, I won’t buy it.
omoshiroi nara, mitai.
If it is interesting, I want to see it.
Verbs: Plain form + なら [You can also use の: Plain form + の + なら]
anata ga iku nara, watashi mo ikimasu.
If you are going, I’ll go too.
chokore-to o taberu nara, ha o migakinasai.
If you are going to eat chocolate, brush your teeth.
okane ga aru nara, terebi o kainasai.
If you have money, go ahead and buy a TV.
Also here are a few common usages with nouns:
anata nara, dekiru.
If it is you (doing it), you can do it. (it can be done)
otona nara, sonna koto wa shinai.
An adult wouldn’t do such a thing.
(if it is an adult, the adult wouldn’t do such a thing.)