Smogon is probably the quickest way to find out what's viable, though it varies on what you're playing. Anything official for example will be different since there aren't tiers, and a lot of the guides there are based around those tiers. But it's a starting point at least. Through that you can get an idea of what works in competitive and come up with your own ideas when you're playing online. Copying smogon sets won't really get you anywhere if you don't understand team composition, so try not to do that.
