populate form onLoad

Hello All,

I want to populate a form onLoad so that the number of contacts linked to an account is displayed. I am unsure how to do this.
I am a novice to using CRM, and am unsure whether to use a workflow, JavaScript, or some other method I'm unfamiliar with.

I wish to do the actual work myself, but any help/advice on what to use/do to make this work would help a lot.
I've already scoured the internet for help, but people are decidedly unhelpful.