|The only requirements to start a jump are that your ship's bow be facing the jump target, that you have enough fuel to reach your destination, that your jump drive is charged, and that you are not within range of an enemy interdictor. Once you have selected a target and are ready to jump, hit "J" in either the tactical or the navigational screen and an "alignment" indicator will show you how long it will take to face your target. Once the timer is up, the jump will begin and the indicator will switch to show you how long it will take you to reach your target.
When you leave your jump, you will exit at the same speed at which you entered. The faster you are going when you start your jump determines how far away you will arrive from your target, ensuring that you have plenty of time to get out of the way should you be jumping toward a planet, then end up cruising toward it at 30 gu/second.