I created a field to put credit card numbers into, but for some reason the field is not staying stationary, (sometimes its sitting in front of or behind another field). Is there a way to lock it in place so it stays in on place in relation to the other fields?
Does the field move depending on the size of the window you are viewing your layout in?
Sounds like you might need to tweak the layout object anchors you have set. Highlight the field and check the "auto-resizing" features found on the Position tab on the Inspector in FileMaker Pro and adjust them according based on what you want your field to do.
Look for anchors in Inspector->Position->Autosizing.
Other that the Inspector->Position->Autosizing stated by Torsten, make sure that you don't have your fields superposed.
If your fields are above each other, the selected field will always go to the top
Do not make your fields overlap
use a popover in which you can put your credit card numbers field => it will appear on the top