how can i fit a outside tap to this?

Hi The issue you have here is any external pipework needs to be protected from freezing in the winter months. Whilst a tap could be fitted to the 15mm exposed pipe which currently has a push fit stop end on, there is an abundance of pipework liable to freezing. My advice would be to purchase a 'through the wall' outside tap and pipe assembly. The pipe assembly part is available in differing lengths to suit the depth of wall to go through (cavity size etc). The water conection is made internally with the external piece being a brass securing plate, where the tap screws into once the thread has been sealed with suitable PTFE or the like. You should also fit an isolation valve at the inside connection in order to isolate the tap should the need arise.

Easy for us to guide you to You tube, plenty of tutorials there on how to install a garden tap. Just select what information is relevant to this particular situation, as you already have the external cold water feed in place. Don't forget to isolate the mains cold water stop cock first. If it has been some time since it was turned off then it may be partially seized, or if you are indeed able to turn it off then we have to hope its not passing, then it will require replacing or fitting a secondary one instead. This work will certainly entail a professional plumber as the main cold water will have to be isolated in the street (Worse scenario)