I agree with Al, the cover letter is not even secondary. It's way down the list. MEET with them, talk to them, discuss their business as well as your own desire to work in the industry.
Interviews are two way streets. There are brokers and companies that I wouldn't want to work for.
Real estate is a sales and relational business. Can you understand a buyers motivation, a seller's goals and an agents bluff? Are you willing to work 7 days a week, market yourself in busy and slow times and set yourself apart from the crowd?
This is not a job, it's a self employed profession and personal business. You may get lots of offers, do you know which ones may be better than others? It's not about commission splits, but about training, support, guidance, protection and innovation.
Don't take the first offer you get. Meet with 3 or more offices and talk to some of the other agents about what they like and don't like about the company, policies and the broker.
Best of luck.