Avocado?!

So, I'm new to these delicious things. But I have a couple questions..

Some that I open are beautiful green, some have brown. Should I not eat or use these?
Also, is there a way to tell when I'm in the store (without cutting them open), which ones are good?
Should I use them right away or can I store them in the fridge? if so, how long will they keep?
Also, mine with the brown spots had some stringy stuff, is that normal?