Right Click in Sources section -> Add -> Video Capture Device while the webcam is plugged in. It may be labeled as USB Camera in the drop down menu, so be aware of that :p
=Begin IMO=
I guess to answer the second question, just try without it at first; get used to just talking to an audience. As far as I've tried, this can build confidence as you keep going and get more comfortable; once you feel confident enough then you can add a webcam in. Lot's of people started without webcams before moving on (see PewDiePie's early vids, for example). You're gonna have people that make fun of you, that's just the nature of the internet, but how you handle it is what's gonna matter.
=End IMO=