Netiquitte says that captializing everything is shouting...
To answer your question, user level security as JeremyNYC suggests is your best bet. He recommended reading up on it because it is an involved topic... A lot to know not too terribly complex. However, user level security will control who can do what in a database and not address the simultaneous problem. If you genuinely want to prevent others from changing data while you are in the file, you will need to open all the tables with locks... Although this is cumbersome programmatically it would work. I don't know the code for locking by heart, so check out help. The real trick will be unlocking the data for you to make a change. I recommend user level security.