Consider purchasing software.
Programs like Hold Em Manager and Poker Tracker have many benefits. First and foremost, they contain a customizable head's up display feature (HUD) that gives information about one's opponents by tracking all of the hands they have played together. There are numerous options for which data to display, which is an important feature, as a player specializing in head's up cash games will want to have different stats displayed than a hyper-turbo sit'n go player.
Another great aspect for beginning online players in programs like Hold'em Manager is their hand history replayer. While a lot of poker sites have a hand history tool, they lack much of the functionality available in other programs. The ability to replay a cash session or entire tournament away from the tables is an invaluable study tool for any poker player.
Sit 'n Go Wizard is an important learning tool for tournament players. It allows one to analyze hands both from the perspective of Chip EV and ICM (independent chip model). It's a great tool for improving one's calling and shoving ranges. This tool is a must for those learning how to win online poker tournaments.