Practice hanging on the bar with both grips
The first thing you need to improve upon is your grip strength to be able to hold your body weight. Working on your pull up is a lengthy process and will take time to develop. So at the beginning of your pull up workout start with hanging on the bars and engaging your lats. Perform this for 3 sets of 30 seconds and work you way up with the time until you can comfortably hold it for a minute. 
Work on the muscles used in a pull up
The muscles used in a pull up are Latissimus Dorsi, Biceps and the core. Start working these in your strength session by doing exercises such as lat pull down, rowing movements and bicep curls. Start with a weight that you can easily to 12 reputation with then start to build up the weight as you progress. Gradually you will start to see the weight you are pulling on the lat pull down machine to be close to your body weight. This is when you will start to see your ability to perform some reps with a pull up or chin up.
Practice the movement with a strength band
If you feel like you are making progress with your muscles used in a pull up and you can hold yourself on a bar. This is when you’ll want to use a strength band to take some of your weight away whilst you practice the technique of pull up. This will also build muscle just like using a lat pull down machine. However, it is more realistic to your goals of being able to pull yourself up.
Engage your core muscles
During the pulling phase you have to engage your core to be able to make sure your legs aren’t just dangling. Core strength also makes sure your centre of gravity is in the right place when you pull yourself up. Therefore, working on core related exercises like leg lifts and hanging leg lifts are beneficial to improving the pull up. However, starting out you will want to perform some simple movements. For example, isometric planks, sit ups and side planks. Once you’ve built up strength in your core you can start programming in more advanced movements.
Strip some fat
One of the things that will help with pulling yourself up is the amount of fat you have on your body. If you carry an excess you will find it harder. However, if you get rid of some fat it will be easier to pull yourself up as you practice all the other points at the same time. There is a benefit to holding more fat whilst you start your practice. At the beginning of your pull up journey the more fat you hold the harder it will be, which means you will gain the strength to pull yourself up at your starting weight. If you don’t get rid of the fat straight away you will feel a lot stronger once you do.
Main tip
As a personal trainer in Ascot and Sunningdale my main tip is to never stop practicing. This is the best advice you will receive because once you stop it is harder to restart. Don’t get disheartened if you’re not able to do it straight away. Make a plan by yourself or with your trainer and start to implement it.