First, congratulations on the CCNA certification.
To connect two offices, you need to be sure that the data passing between them is secure from interception, or corruption by a third party.
To achieve this you can either get a leased line from the Telco, which is a private circuit, only for your use. Or you can use a VPN connection, which is a virtual circuit that can go over a public network as the data is encrypted to make it safe.
If there is not too much data passing between the two sites, the VPN is preferred. It is much cheaper to use and is fairly easy to set up once you have the Internet connections. It is the same cost whether the two ends are next door, or on opposite sides of the world. You really only pay for the local connections, and nothing for the bit in the middle (the Internet !). However you do share it with everyone else, so the bandwidth is not guaranteed.
A leased line costs for every mile (or kilometre), plus a heavy price for the end terminations, and a heavy rental cost. It does have the advantage of very little config needed to get it working, and you can choose the bandwidth you need, and it is always there for your use only. There are similar options, such as MPLS or Frame-Relay, which give a virtual leased line, at a lower cost. If you want to go down this route, you need to speak with your Telco to discuss the options.
Both need some equipment at each end, routers for the leased line, usually firewalls for the VPN. If you want to stick with Cisco, and the companies are small, then the ASA5505 will do the VPN, or 1800 routers for the leased line. If they are bigger, then choose the more powerful routers or ASA models.
You already know how to do the leased line, it was part of the CCNA, the VPN is a bit more difficult, but the basic config is not too bad. Suitable configs for each solution can be found on the Cisco website.
If you need more pointers, please ask here.