OK, you've found the best site for you to play at, the best game, and the best level. Now you've got to find the best table. If it's a tournament, obviously you won't have any table selection available, , so we're addressing cash games here of course.
First, let's look at what we're after here generally. As a rule, we need to prefer games which are as loose as we can find, while at the same time have as high an average pot size as possible. If you're a newer player, it pays to pay more attention to things like percentages on the flop, and as you get better you can focus more on getting in the biggest games you can in terms of pot size. The reason for this is that high average pot size games tend to be more aggressive, and while these games are the juiciest, they also tend to be more aggressive than average. So until you feel comfortable in a game like that, with more betting and raising by opponents than usual, you should select on the basis of looseness of the table only. The looser the table, the worse the players, and this in particular is what you're looking for at this stage of your development. Ideally, we want them to play wildly throughout the hand though, so ultimately the table being both loose and aggressive is what we're really after when we are ready to handle this type of action effectively.
Most sites have the flop percentage displayed in the lobby on each table, and all have the average pot sizes. We're going to use this as a guide to direct us to tables which have good potential for earnings. You can also use advanced tools like Poker Edge's Lobby Edge which will search across multiple sites and automatically select the easiest tables to win at. This is an amazing tool by the way and can make a huge difference and especially since there's no real way you could do this on your own.
For starters, flop percentages displayed in the lobby are based upon the action over the last few hands, and while this can give you somewhat of an idea of how loose the table is, it doesn't tell anywhere near as good a story as Poker Edge does in this regard, because he is going to give you the flop average of the table based on the play of the players there over a much longer period of time. In fact Poker Edge takes account all the stats he has on all the players in coming up with this figure for you, so it's going to be a lot more accurate and reliable.
Another benefit of this is that flop stats in the lobby are only updated periodically. People tend to move in and out of seats pretty fast online, and often a table's characteristics can change significantly from the last time the lobby stat was compiled. For instance, let's say a table is very loose. Players in the know like yourself spot this, and join the waiting list. You join it as well. By the time you get a chance to sit down, the table may be populated not by the loose types that were there originally, they may be more populated now by sharks as yourself who are looking to prey on these fish. So instead of a table of fish, you've pretty much got a table of sharks now. Since Poker Edge bases its stats on the overall looseness or tightness over time of the actual players who are at the table right now, this will save you from getting involved in these situations when you don't want to, by alerting you to the actual state of the table, in exact fashion, at any given time and at any given table.
In addition, what can happen through the influx and outflux of changing players is that what was once a nice and loose table can often tighten up on you. You want to be alert to this of course, and while you could keep checking back in the lobby for this, it can become a nuisance, in addition to the information being outdated and not particularly accurate. Throughout the game though, all you need to do is look at Poker Edge's table stats, and you are kept informed of changing circumstances hand by hand, so when it comes time to look for a better table to play at, you'll know exactly when your criteria for this has been reached.
As well as looking at the overall table stats to determine the desirability of a certain table, Poker Edge will of course give you the goods on each player presently sitting at it as well. If the table is populated by mostly tight players, with only one or two fish, you may decide that there's too much competition for the fish's money, and may choose to look elsewhere, where there is a higher proportion of bad players to prey on. Or, you may find that the aggressiveness level is too high for your liking, both in terms of the overall average or too many more aggressive players in total, especially ones that have good position on you, which is to say are close to your left. Whatever your preference for good conditions to play, you can use Poker Edge's capability to assess how desirable or undesirable a certain table is for you, both when you decide to sit down, and throughout the game as well.