Tactical Tips : What to do with your next second

Fighting in GURPS is done in turn of 1 second. Yes, you heard that right, 1 second. You have to think fast or else the GM may just compel you to "Do Nothing" until you figure out what to do. During each turn, a player has a number of tactical actions to choose from. There is dozens of options: you don't need to use or even know them all, but at least, it is good to know that they exist and develop your own fighting style from a subset of them. A great digest of ALL options can be found here. In this tip, I focus on the basics.

Basic Actions that your must know about

In principle, you can go by with only these: Move, Attack, "Do Nothing" and Ready.

• Move : Move your figurine by a number of hexes equal of less than your base move. If anyone attack you, you may defend whichever way that you please.
• Attack : This is the run of the mill combat action (think D&D). With it, you get to pick a target and try to hit as hard as you can. You will still be able to parry, block or dodge normally. The most that you can move is a single hex (usually), which you can use before or after attacking.
• Do Nothing : No offense nor movement, nothing at all. You may defense using active defenses and take free actions such as talking, crying, and picking your nose. Will often be imposed rather than selected.
• Ready : Get something in or out of a sheath, change your grip on a weapon, picking something in reach of your hand (dropping is a free action). More technically, a ready action is required to change the reach of a weapon (if applicable, more on Reach later). Reloading a weapon may require one or more ready actions.
• Change Posture : Stand-up, crouching, sit-down, kneel, crawl, lying-down or prone. You cannot change from lying-down/prone to standing up in one action. Also, crouching is a free action.
• Misc actions: 
• Concentrate : Focus  on some mental task. 
• Wait : Waiting means that you chose to defer your action to a later point in the turn following some kind of trigger. Typically, waiting is used to attack an opponent in the middle of its movement.

Taking this to the next level

There is a lot going on here, but lets just consider the salient points. You can check the details in the linked document above.

• Move and Attack : Go on for up to a full movement and attack (or vice-versa). The Attack will be done with a stiff penalty, and you can't parry and attack with the same hand. But you can slam, and generally take a chance to spoil an opponent's defense for an allies to exploit later.
• All-out Defense : You waive your chance to attack and commit all efforts to defense. It comes in two flavors: two active defense per attack, or an enhanced dodge allowing 1/2 move as withdrawal (which is likely to break contact with your opponent).
• All-out Attack : There are many flavors of the full attack, but the general idea is that you waive any active defense so that you can strike better/harder/from afar/twice/with a feint. This is the thing to do when offense is a better defense, or you know for sure that no one will be trying to hit you for the rest of the turn.
• Feints, beats and ruses : These three variants aims to penalize the opponent's next active defense roll by using your weapon's skill (feints), ST (beats) or IQ (ruse). There is more to it that this one liner, but these maneuvers are useful to defeat opponents with very high active defenses.
• Aiming/Evaluate:  When you aim a ranged weapon, you gain its accuracy rating as a bonus when you shoot. If you aim for 2 turns, your add +1, and another +1 for 3+ turns aiming. As you can see, taking your time pays off when shooting. Bracing a crossbow to a wall, table or the ground gives you another +1. You now know who to give the "Moe Green treatment" to your foes. Evaluate is the melee combat equivalent of aiming: you pause and look for the best angle and timing: You get +1 per turn examining up to a bonus of +3. 

Beyond these, there is a lot more to consider. However, I'd be tempted to say that the full range of actions are there for you to develop a personal style.