Your first step should positioning yourself under the pull up bar. Most beginners make the mistake of grabbing the bar in a leaning position. Remember, your feet should be right under the bar. When you extend your arms to grab the bar, your entire body should be straight with your eyes on the bar.
Your next step is to position your arms in line with your shoulders. In other words, your palms should be shoulder width apart. There are variations of placing your palms. The wider you take them the more difficult the entire motion will become. For beginners, it is best to keep your palms positioned along with your shoulders.
Now, you need to use your arms to lift your entire body up. Make sure you are using your strength and you are not jumping off the floor, a mistake most beginners make. It will be tough and there are chances that you will not accomplish the task in the first go but do not let it change your form. Form will build your strength for better pull ups next time.
Once you have lifted your body, go up until your chest is against your bar. There is no hard and fast rule here. Your upper chest should hit your palms. If you have maintained the right form, this will happen without any additional efforts.