Consider looking at the following Credentialing Guide . This guide is provided by USW 5000 union and has some responses to your questions as to the USCG application process.
It also speaks to possible employment and employers on the Great Lakes.
As an unlicensed entry level mariner, Great Lakes employment does not require STCW endorsements which are required by deep sea vessel.
However Great Lakes or other shipboard employment will get your feet wet.
Once you are employed, then take the next step to start look into National and International STCW training and endorsements and the required sea time needed for advancement. Keep in min this training and MMC endorsements will have associated cost. So save up some money for upgrading to the next position.
If you want to be in this industry, you will need sea time, tonnage and experience on a vessel in a ship’s department (deck, engineering, or steward) pick the one that you feel represents the best path for your future.
Good Luck on your future choice.