All the poker books point out the value of choosing a profitable (or "hot") table. I totally agree, but it will get your night off to a slow start. It takes some time to play spectator in order to see the skill and aggression levels of your potential opponents. That's what's nice about on-line table selection, I have some tips to speed up the process.
Before you start playing, most poker sites have a display of available tables. Each table often includes stats such as the number of seats available, how many seats are taken, the number of players on the waiting list, the stakes and the average pot size. Average pot size is very important and one stat that you can't immediately gain by visiting a table in a casino.
You should, by now, know thyself. If you play your best game shorthanded, stay at the tables with a max. player limit--usually six. If you're best heads-up, there are table for that too. On-line is great for options. Are you a tournament player who usually either goes out first or wins it all? Then you are probably aggressive enough to be suited for a "turbo" game where the blinds raise quickly. If you like to wait for the really good hands, stay as far from turbo as possible. Some sites even have games with extended blind levels, those may be more your speed.
Another factor for aggression is average pot size. For easy money, an aggressive player should steal the blinds of the table with the smallest pot sizes relative to their blinds. A tight player should sit at the table with the highest pot sizes so that when you do get your hand, you can win big. This strategy probably won't be the most fun for either player type, but it will be the most profitable.
The time of day can also be a factor. I have found that European players are overall a different skill level as US players and through the magic of time zones they play when we sleep. I'm not saying which countries host the most skilled...but I have my theories.
Keep in mind, that just because the table stats say one thing, doesn't mean that will hold true forever. Don't base future play on that early information. A aggressive table can quickly turn tight in the event that the bully loses his chips and is replaced. I'm just saying...test the waters before you jump in.