I haven’t figured out how to do a link very well, so the “long” way to get to the video you want is to click on the Basic Techniques tab at the top of this page, then click on “…AND MORE”, scroll down to the fifth picture and you will see “Joining the same color yarn”. That video is your answer.
As far as amounts of yarn, it depends on the pattern. What you should do is go to the free patterns tab above and browse the scarf section. That will give you a better idea of what is available and supplies you might need.
Most likely some kind person will come along shortly and pop a link in here for you, but if you’re like me, you want an answer NOW.