Do you mean hardware or software?
I would start from a software point of view. So first, as a traveler, I’d go with antivirus and VPN (protection is very important). Then I would go with a more specific software based on your agency needs. And of course a WiFi device.
For software, I think the most important tools are:
1. A browser to login into the webbased order pages for travel agents with the tour operators, car rental companies and other companies he sells services of.
2. An ERP-like system for invoicing, payments and bookkeeping, probably also some CRM to keep track of what his customers do.
3. Depending on what his customers are an option for them (like a chat on his website, or a business account for Whatsapp) to contact him.
There are a variety of technologies that can be used to help run your business. Some popular tools include reservation systems, which can be used to book flights, hotels, and rental cars for clients. You can also use customer relationship management (CRM) software to manage your contacts and interactions with clients.