I believe the method mentioned by agif can be done through the following:
Go to the form design view, then right-click in the grey box in the upper left corner where the vert. and horiz. rulers meet and select properties. This should open the form properties window. If a window goes away, repeat the step to bring it back. In the form properties window, click the event tab. Scroll down in that box until you find the "On Load" event, click in the text box next to it, then click the ellipsis (sp?) button. Select Code Builder and click OK. You should then be in the VBA coding environment. Once here, type in "Me.Form.AllowEdits=False" (without quotes). That should be all for the form. Save and close the VBA window (its separate from Access, so you won't close Access when you close the coding window).
Similarly for your command button, go back to design view for the form. If the properties window isn't open, right click your button that you plan to use to enable editing and click properties. Go to the event tab and find the "On Click" event, click in the text box next to it, click the ellipsis button, choose code builder and click OK. You should be back in the VBA coding environment again. All you should need to do is type "Me.Form.AllowEdits = True" (without quotes). After this feel free to actually open your form and text it out to see if it works.
Hopefully from here you can understand how to write the code to another button to disable the fields if you choose to put one in! And sorry for the long post, but I'm hoping this will be thorough enough to answer any questions you may have about where to go.