The last person knows how many opponents they’ll be facing, they know if there were any raises, or any re-raises. If you’re acting last, you’ve seen everyone make their play. You have an idea of how people feel about their hands. The first person to act has no idea. They have to make their decision before they see any else’s actions.
The general rule of thumb is to play tighter (be more selective in your starting hands) in early position. That requires you at least be aware of your position.
I’m going to refrain from spouting off situations and actions, like “in early position raise with this, call with that, fold with the other”, etc. If you want a blueprint like that, I’ll once again recommend Lee Jones’ “Winning Low Limit Holdem”.
Besides being aware of where you are in relation to the button, it’s also best to be aware of the way the other people at the table play cards. Even though online poker resembles a video game, those are actually other people you are playing against. Pick up their tendencies.
Since most holdem ring games go “fold, fold, fold, fold . . . ” there is plenty of time to pay attention to what’s happening when you are out of a hand. If someone raised pre-flop, pay attention to what cards he had (if you can see them at the showdown). Pay attention to who raises with only strong cards, and who raises because they’re bored.
Take the information you learn from watching people, and use it to make more informed decisions. Poker is a highly “situational” game, so what may work at one table of opponents will not work against another.
The crux of this whole page may be Pay Attention, but I think that’s worth saying. One of the larger holes in my game came when I stopped paying attention to the players I was facing at the table, and treated all $1/$2 limit games, or all $20 tournaments the same. And that’s not the best way to approach things. Each table is different, and you need to play the players at the table to play your best poker.