I need advice on minecraft?

I'm very new to minecraft - just got it yesterday - and I dont' know what the overall point of minecraft is. It doesn't seem there are any concrete goals in singleplayer. I just work to get better minerals and armor and stuff, but what's the point? I want to get on multiplayer to really experience the open-ended quality of minecraft, and I've gotten on a server, but I have no idea what to do. I couldn't mine any materials (everything would just regenerate and I would get nothing from it), and people were trading items and money as i could tell from the chat, but I didn't know how to do any of that. Any advice?