Chicken stock is used almost everywhere because you can buy decent chicken stock. The FDA requirements for beef stock require it only to be brown, and manufacturers are happy to oblige; keep away from it. Good, homemade stock is what separates real cooks from wannabees, but in the meantime invest in a few jars of Better 'n Bullion. It is an honest product, exactly what it says it is, and it comes in chicken, beef, ham, seafood, mushroom styles.
Onions are another fundamental flavour-group. My suggestion is to just play with them, find out what you like. Green onions are best as an almost-raw garnish, while standard yellow onions can do astonishing things like turn into candy when you caramelize them, which only takes a slow cooker, a lump of butter, a dash of salt, and a couple of hours.
A perfect (and fun) example of treating onions differently comes from Keith Richard's autobiography, Life
. Given the fact that he doesn't sleep like normal folks and therefore has to cook for himself, he came up with his own version of Shepherd's Pie. His big gimmick is to use browned onions AND raw onions when assembling the pie, and the result is fantastic.