Before you write your letter try to contact the person in charge by email or telephone, even face to face, to make them aware of who they are. If you explain your situation to them briefly they may be able to give you a better idea about their need for photography services. If they state outright that there is no way that they need a photographer, it is still worth writing but do word the letter carefully. If they suggest that they are interest then you should write to them as soon as you get home.
Header the letter with your name and address. Add any other contact details that you feel are appropriate. If you have a website, include this in the header somewhere. Date the letter so that you can refer back to it should you need to in the future. When addressing the individual try to use their name if you have already met them or if not use 'To the manager' or whoever else you think is correct. If you have already spoken to the person begin the letter with 'After our conversation on.....'. Here you can either mention that the manager said they were interested or if they weren't you can mention that you know this is the case but thought they may still want to keep your details on file. Briefly summarize the type of photography and what your charges are. It is important to mention, particularly if you are just starting out, that you can be flexible. A fixed charge that the manager feels is too expensive may put you bottom of the pile. Keep it short and sweet and include either a link to your portfolio online or include a few photographs. Make sure to mention that you can bring a full hard copy of your portfolio should they want you to.