When to play certain champs? Example: Viegar

So Ive been playing league for a little under half a year, so not that long. I still have trouble with picks and counter picks! For midlaner most part, i understand how some champs are better against others. For example, I know i should run zed usually against a squishy team with no real tank and/or a mage in the mid lane. Also know Twisted fate is a great counter to assassins with his stun and other abilities. I recently got viegar, saw youtube vid of people dominating with him, so why not? I understand him, and can get great stacks as im good at q farming, but when should i play viegar? What i man to say is, who does he counter? I know a lot better to play him against most assassins, but besides that i dont really know anything else about his counters or those he counters!

Also if you have any good midlaners youd recommend Id love to hear them! I just finished messing around as adc and a tp laner, and have a large champ pool for most lanes except mid.