A question often of newbies. The green dots on the Mini-Map indicate people.
An Infohub, also known as a Welcome Area, is a place where Residents congregate and socialize. Click Search > Places tab and search for "infohub" (it's not CaSe-sensitive). This will show most of the Infohubs.
Some areas where there are to be found a lot of people:
Ahern Welcome Area http://slurl.com/secondlife/Ahern/4/4/40
Sweethears Jazz Dance Club http://slurl.com/secondlife/Sweethearts/197/148/23
Phat Cat's Jazz Club http://slurl.com/secondlife/PhatLand/170/104/22
Other ideas for finding people include:
Search for events ('Search' button at the bottom of the screen followed by the 'Events' tab along the top of the window; search categories include discussion and live music).
A geographically based group will often announce events.