You could start by applying to law firm events, open days and insight days to experience what it is like to work at that law firm. This is a great way to discover whether the work that the firm does interests you and whether you like the firm's culutre. It is also a good way to build a relationship with that firm when you will later apply for vacation schemes/training contracts. Make sure your applications stand out and you have very good answers to the questions "Why commercial law?", "Why the firm?", and "Why you?". If I remember correctly, Commercial Law Academy has a cours on writing successful applications and law firm profiles.