Depends on the type of bots you want to add. There are mainly two types, ZBots and PODBots. Download either of them. I would recommend ZBots, a bit more faster and intelligent but does not have the...
You can add bots in many different ways, it depends on the AMX plug-in that you are running. hopefully, you are using PoD bots and the command is amx_podbot add #.
If it's your own server, it's best to start by inviting your friends onto the server. If your server shows up in the public server list, people may also join in on their own.