Welcome!

It depends, you might like to get the Job for that second class at a later date, in which case it is definitely worth levelling. Not to mention levelling other classes gives more abilities to cross class.

As for how to level, your first class you can pretty much take from 1 to 50 (the current final level cap) by pretty much just completing main story quests. There are other ways of levelling as well, such as the Hunting Log, FATEs, guildleves, and even dungeons all give exp, which if you have the exp boost for levelling a second class that is lower than your first one will make things a bit easier.