Hi there,
What you are trying to do is not an exact science and it really depends on the session and the cards your opponents get. All I try to do when I play is establish betting patterns and see how often players call and raise. The most information can be obtained when you actually see a hand at a showdown and can replay it to establish a betting pattern. From there I give a colour to the players at the table and make notes on them like; bets 3 x BB on Ax hands or bet sizes vary with the strength of hole cards. My colours tell me if a player is tight, aggressive, passive, loose, tight-aggressive, or loose-aggressive. If I cannot classify a player I simply give a neutral colour and note their general performance during the session. That way I know how well they did and if they took money from me!
If you are playing live, then you simply make a mental note of the aforementioned tendencies. If you never get to see a player's cards because their opponents fold to their aggressive betting then you simply have to avoid them and be patient - wait for a huge flop or very good hole cards AA, KK, AK or suited connectors if that is your cup of tea.
Finally, if keeping track of your opponents is difficult then keep track of your table image and how your encounter with each player is handled. Try to get an idea of who generally calls your bets, folds to you or raises when they play against you. When I play, it is a mix but I generally try to maximize my position and would usually come into a hand with a standard 3 x BB and c-bet or check-raise, especially against aggressive players. It has been my experience that you get more action when others see you as a controlled player willing to play a wide range of hands and not just huge holdings. I am usually the chatty one at the poker table and always try to get a reaction from players while keeping the game fun.
Playing live poker is a lot of fun and I prefer it to online play. I get a much better read on the person's personality and their tells.