Identify date of invention
You don't actually need your date of invention to file an application. That date will be very important if you are ever involved in litigation, so you may as well figure it out now - or at least narrow it down to a range of dates. Save receipts for materials you purchased to make any prototypes. You do not need a working (or even a non-working) prototype in order to file a patent application.
Make notes for patent attorney about making and using the invention
The application will need to describe to the USPTO (Patent and Trademark Office) how to make and use the invention. The application needs to enable a person of skill in your field to make and use your invention. There may be more than one way to make and use your invention. If so, elaborate on those.
Make notes for patent attorney about best mode of using the invention
There may be particular aspects of the invention that you know about and are not easy to figure out. For example, the invention may work better at certain temperatures, when positioned at certain angles, if made of certain materials, etc. Care needs to be taken to discuss these with the patent attorney so they may be included in the application for patent.
Decide on utility application vs provisional application
A provisional application for patent is particularly useful if you need or want time - time to tweak or improve the invention, arrange for marketing, arrange for distribution, arrange for manufacturing, raise capital, attract investors, etc. Discuss the type of patent application you should file with your patent attorney.
Keep everything secret until you file the proper application
Selling, or even offering to sell, the invention call cause you to lose your rights to get a patent. So can disclosing the invention or even using it. Better safe than sorry here.
Identify other possible inventors
Not every person that helped with the invention is necessarily an inventor. If somebody just supplied materials or followed your directions, they are likely not inventors. Make a list of every person that helped with the invention or that might think they are an inventor and give the list to your patent attorney. Intentionally leaving off an inventor can lead to the patent being declared invalid.
Make some drawings
Just about all patent applications require drawings for the USPTO to understand the invention. You can have them done professionally before filing if you want them to look good. There are specific drawing rules about how thick the lines in the drawings should be, how shading should look for curved surfaces (and a bunch more) so either learn those rules, or have the patent attorney suggest a draftsman prepare the final drawings.