Use PuTTY or another terminal client to connect.
If you have the login details and know the IP address, you may be able to login over the network via telnet or ssh.
If you don't know the IP address, and this is in use already, you will need to connect via the console cable.
Note that a console cable will have come with the router (or one like it). For most Cisco routers this will be a blue cable with an RJ45 connector on one end, and a serial connector on the other. If you are using a laptop with no COM port, then you will need a USB to serial adapter.
If you don't know the login details, and are prompted for them, then you won't be able to get in without resetting the router, which will knock everyone connecting through it offline.
Once you are in issue the command
show version to see what version of the operating system is running, and details of the exact model you are dealing with. Then search on the Cisco site for the manuals corresponding with that.
If you are a total beginner and anticipate doing more of this, do the Cisco Certified Network Associate (CCNA) course. A great investment. If this is just a one off, then hire someone to do the config for you.