Line spacing and not showing sidebar



I am building a blog & portfolio using jekyll. I am not able to figure out how to control line spacing using jekyll. My blog posts look rather crammed right now. Following is the link:

One can notice that there is no vertical space between the lines. Also, I would like for the sidebar to not show up in a post. I like blogs who have content aligned vertically right through the center of the page with empty spaces on left and right.

Can anybody tell me how can I possibly go about tweaking things to achieve this?

P.S: I am a Statistics major, forgive me if my question is too basic.