Its always great to see people work so hard on their pull ups and then getting their first one. Its like watching the birth of a butterfly.
However, what you'll often see when people struggle through that first pull up is, to put it bluntly, a pretty ugly attempt and a horrible body position and movement mechanic - Overarching, breaking through the spine and some horrible kip style movement.
Whilst this is quickly forgotten about after the cheers and bell ringing, its something that needs to be thought about long after your first or even 20th pull up.
The key to getting injured is moving with poor mechanics. Losing the tight hollow position will ensure your body is unable to create strong stable positions in other areas like the shoulder.
Staying tight in your movement is key, if this isn't achieved your body will begin to move very inefficiently and look for stability in unstable positions (i.e knees collapsing in on your squat is your body looking for stability in an internally rotated position - however a more stable, stronger position is the external rotation of the femur - cue.. knees out!)
If theres one good thing you do today, besides drinking great coffee, its to watch this video and start thinking about how you move when you do your pull ups.