New to CRM & business management – need some advice

In short, I'm helping someone as a VA with their small business. They rent out industrial equipment to customers. My client has 4 of these machines and has, for some reason, created a separate gmail account for each machine – and in turn they all have their own calendar for bookings. Their website has a Calendly plug-in but the whole system is just chaotic and doesn't make a lot of sense to me. I've told him it's definitely out of my expertise but he wants me to work with him on it regardless. His leads are currently in an excel spreadsheet. I just want a platform to streamline all of this but it feels very intimidating. Any advice?