Tasked with implementing a firewall into a small business. Need help.

So to make a long story short, I work at a smallish company (around 200+ users total) in the IT department (first real IT job) and I support our users who are here at our office and also some located in our New Mexico office. Our security is terrible, nearly non existent. We do not have an Active Directory, nor do we have a firewall. Passwords are all kept in Keeper and we have soooooooo many user credentials its an absolute nightmare how our data is managed. My IT lead is concerned w security and saw that I am studying for my CompTIA Sec+ certification, so he tasked me with coming up with some ideas to increase security. He definitely agrees we need a firewall and he has tasked me with figuring out how to implement one, but I do not have the slightest idea where to even start. Don't know how many ports I need on the firewall, don't know what kind to buy, or how to even configure it. My networking knowledge is rather entry level, so this is quite a daunting task. If anyone has any suggestions or can help me in any way please reach out to me on this thread. Thank you for anyone that decided to help me out :)