A large part of Star Wars: The Old Republic involves player characters engaging in conflict with NPCs (PVE) or other player characters (PVP). The specific separate enagement that maybe occur between one or more player characters or NPCs, typically resolved by reducing characters health to 0, or until they surrender, is referred to as Combat.
The in-game Codex has a listing under Game Rules that is unlocked during the early startup of a new player character. It covers several aspects of Combat within the game.
You can attack enemies by left clicking on them, moving into range, and using an ability that engages the target. The Action bar, typically at the bottom of your screen during startup, gives visual indication of when you are in range, by lighting up the abilities that can be used on the target.
To view your available abilities, you can open the Abilities panel. There is an icon on the main Menu bar, typically at the top of the screen, that will open the Abilities panel, or you can use a hotkey, initially assigned to P. Once you have the abilities panel open you can drag the icon for an Active ability to your Action bar.
After you have defeated a target, you may be able to Loot items from the corpse. The body will highlight in various colors, representing the type of loot it contains, if there is Loot available. You can right-click the body to open the Loot window and right-click on the items within to move them to your inventory.
As you defeat enemies in Combat, you will gain Experience points (XP). As your total XP increases, you will advance through Levels. As you reach new Levels you will want to see your Class trainer to learn new abilities that become available.
Class Combat Styles
Each of the different character Classes, and even especially the Advanced Classes, provides whole new Abilities and Skills that ultimately define how a character engages in Combat. You should explore the details of the classes to learn more about the combat for your specific character. A common aspect of games such as Star Wars: The Old Republic is the various combat roles, such as tank, healer, or damage dealer (commonly listed as DPS for damage per second), that a character may fill. As you select your Advanced Class for you character, you should evaluate the potential role you would prefer to fill when engaged in Combat as will help guide selection of the available choices.