As others have said, there is obviously a big difference between live poker and
online poker, since in live poker you can get information from peoples physical apperance, their table talk and the way, they conduct themselfes on the table. I know nothing about this, since I am an online player, and I will assume, that so are you. So from now on I will only focus on online poker.
I will also assume, you are fairly new to poker, because otherwise you would hardly ask a question like this in "learning poker". And the main thing, I will say, is, that its far more important for you to work of your baseline or default strategy than to work on observing your opponents. We have all seen TV-
poker hands, where Tow Dwan pull off some sick
bluff, or Daniel Negreanu make a huge soul read.
But the vast majority of the time the best strategy in poker is to play almost like, we are a robot rather than trying to guess, what our opponent might be doing. Some call this GTO, others call it ABC or TAG. Some part of this is knowing our preflop ranges, which boards to C-bet more or less, when to continue on the turn, and how to defend somewhat correctly against aggression.
So if everyone is an unknown, because a new tournament just began, then we just settle in and focus on playing our hands in a straight forward way and stay out of truble. And only when the blinds have gone up, and perhaps we have more information as well, do we start to make more adjustments and fight more for the blinds and antes. Its an old truth, that you can lose a tournament in the early game, but you can not win it.
But if you are going to pay attention to, what your opponents are doing, then the most important thing is to look for unusual action like very large or very small bets. If for instance someone straight out open to 5BB in an online game, where the normal sizing is 2-3BB, it is usefull to know, if this was their first time doing it, or if they do it all the time. If its the first time, we can just fold anything but very strong hands, but if they do it all the time, we should start widering our range, because they likely dont always have a premium hand.
Finally I will say, that the most valuable way to observe your opponents online is by using a tracker / HUD, if the site allow it. In that way you will often find, that you actually do have information about some of the opponents, because you have played with them before. Notes and tags are also usefull for having such a "memory" from game to game.