Bleed is usually 1/8" in the US. Set that when you create a new document in InDesign. Once you've then got your guides, master page and any style sheets sorted out, save the document as an InDesign template under the save as dialogue.
The only difference between a template and a document is that you can't save over a template. When you have a template open and work with it, adding new material and then save it, it asks you to give it a new name for a new file. Try it with a blank page to see how InDesign treats the two kinds of files.
If you are still not sure, take a sample of your card to the post office. They can check it and see if it meets the requirements for mailing. Should not cost you anything for them to do this.