Take hold of the two bars at your side and lower yourself till your arm make 90 degrees and bring yourself back up then repeat this nice and slowly to get maximum benefit. If you find it too hard to lift your own body weight the there should be a seat for your knees underneath you so you could place your knees on there and once your more comfortable and ready then you could try again without the seat. If you find it easy to lift your own body weights why not attach a weight around your waist like 5kg or more depending on how comfortable you feel.
If you are starting out then you could do:
1st set use the seat to hold your knees and do 10 reps (beginner)
2nd set 10 reps again
3rd set do 5 reps by yourself.
Move up reps every week and keep adding weight.
[image from workoutlabs.com]