You need to know before you do anything what a model has to do to make a living. Research this properly, don't just call agents and waste their time asking a thousand questions. It's important that you know what to you are talking about.
Agents don't like surprise visits, they prefer you make appointments to see them. Always call an agency first and ask them what their application procedure is. Never send pictures without your stats (height & body measurements) if you call them and they ask you what your measurements are and you don't know you will sound inexperienced and under researched and this won't impress anyone.
Always include a contact number with your photos so an agent can get back to you fast. There may be a job in that day that they might want you for so don't make it hard for them to contact you. Always include a stamped addressed envelope with the correct postage for a response and the return of photos.
Research Tools
Books - go to your library and read books on how to be a model.
Internet - check agency websites and advice pages.
Advice Lines - some agents often have advice telephone lines but be prepared to pay heavily for the calls.
Newspapers - The stage and other newspapers may help with advice but be aware of dodgy modelling schools that advertise.