Welcome to Malifaux! The hiring rules are:
You can hire any model that shares a keyword with your leader for its printed cost, regardless of faction. Your leader is free. If your leader is a master, their totem is free. If your leader is a henchman, then you get your leader's faction's effigy for free.
You can hire any model in your leader's faction for its printed cost +1. Sometimes you'll see this referred to as the "tax" or "OOK tax." You can even hire additional masters.
Finally, you can hire any model in your leader's faction that has Versatile for its base cost, without paying the tax.
When you're just starting out, you will probably hire a lot of keyword models. But as your model knowledge and playstyle develop, you may find OOK (out of keyword) or Versatile models are a better fit for what you are trying to do.
At the risk of self-promotion, we have a podcast whose first few episodes are newer player focused: https://www.theothercoastpodcast.com/