It's going to be service-specific, and will largely vary within services depending on the operational community you're a part of.
So here's what I'll try to tackle, first tour, junior officer of a U.S. Coast Guard cutter. I'm basing it on my experience.
When thinking about your first tour on a Coast Guard cutter, think about it as life inport, and life underway.
INPORT:
Inport is a period to training. For the Coast Guard, as a junior officer on a cutter you will either be a deck watch officer (DWO) or an Engineering Officer in Training (EOIT). Independent of being a DWO or EOIT, in port you will learn about the ship, sign off different qualifications (such as learning about damage control), and you'll have the fun experience of tracing the fire main.
You will also go to schools inport. Sometimes you may be at school for part of a patrol, but the goal for the command is to get you training while you're inport.
In the Coast Guard you will go to a fire fighting/damage control school, where Navy squids will try to talk down to you and your shipmates until they find out you're smarter than them.
You may go to Boarding Officer or Boarding Team member school at the Federal Law Enforcement Training Center (FLETC) in Charleston, SC.
I went to the Navy Electronic Key Management System (EKMS) manager school in Mayport, Fla., and the Coast Guard Public Affairs Course at the Defense Information School (DINFOS) at Ft. Meade, Md.
EKMS was boring. DINFOS was fun.
In addition to learning about the ship and your primary job as a DWO or EOIT, you also have to learn about your collateral duties (for instance, while I was in a DWO billet, my collateral duties, such as Assistant Operations Officer or Communications Officer are the jobs that allowed me to manage divisions. It was as the COMMO that I had petty officers under me. In that role, you also have to deal with administrative issues, your guys' jobs, leave, etc.
And finally, in port, you will stand watches and work on different jobs to become inport (officer of the day or officer of the deck) OOD qualified. Until you get qualified, you are, in a way, taking up space. They want you in that duty rotation.
UNDERWAY:
Underway you also have things to learn. As a DWO you want to become underway OOD qualified. There will be other things to learn, such as the "rules of the road" for driving a ship, maritime law enforcement, how to use a sextant. When you first get there you'll "break in" under someone who is already qualified. They still have the responsiblity, but you'll be able to get your feet wet under their guidance. As your experience grows you'll take a more active role.
You also have drills on board, ranging from "man overboard drills" to "abandoning ship". You'll have gun shoots. Maybe you'll work with a helicopter, landing and taking off on your flightdeck (if you have one). Maybe you'll work on small boat lowering/raising drills. When there's time, you may have a "swim call" where you get to swim next to the ship.
You still have collateral duties. Underway, when I wasn't on the bridge on watch, I was in my folks' work area, seeing what they were doing, signing stuff off, talking about issues or projects, etc. As a leader you want to set your folks up for success, so making sure they're getting qualified or training or resting or working is important.
My guys dealt with the navigation of the ship, communications equipment, classified material, the systems and electronics and operations. I was usually with the bridge BMs, in the ET shop, or in Combat Information Center/Radio Room.
When I wasn't doing that, I was working on other collateral duties (such as public affairs officer, morale officer, etc.) in my stateroom. A lot of work also gets done in the wardroom.
Underway, when you aren't doing those administrative things, other things are going on. Honestly, often other things are more important and you try to find the time to fit in your other responsibilities. Those other things depend on what you're doing. "Down south" we were doing migrant and drug interdiction. Funny thing was, migrants didn't really care about our schedules, so at any time, even while your plate is stacked high with all of you administrative duties, you have to stop 200 Haitians or 30 Cubans who don't really want to be picked up. In the north, it's a non-stop schedule of fisheries boardings. And no matter where you are, a search and rescue case is always a possibility.
I remember the long days of waking up at 2- 2:30 a.m. to get ready for watch, doing on a round of the ship, checking in with engineering and CIC, and heading to the bridge around 2:45 a.m. or 3 a.m., getting my bearings, getting my night vision, talking to the off-going OOD and relieving the watch. I would then have a four hour watch, and that 4-8 a.m. watch was just not fun. So many things had to happen. Often stuff was going to happen in the morning, such as starting boardings or heading to a migrant boat. You could putze around all night, but by that 4-8 a.m. watch, at some point the captain or OPS was going to be there with directions to start off the day. You'd get off watch around 7:15-7:30 a.m., head down to grab a bite in the wardroom, and then start the work day. And if you were on "Double 4-8s" this routine would happen again until around 7:30 p.m. at which time you'd have some more administrative work to do (since 8 hours of the day were spent on watch), and then, after evening reports, you'd try to get a shower and be in your rack for 5-6 hours until your next round of watches.
Double 4-8s was a hellish schedule, alone. But adding it the other duties you had, and then onboarding migrants or conducting a SAR case, could really beat down your reserves.
Keep in mind, while you're doing this work, your "home" is moving. Nothing like being 30' above the water line in a 210' in 25' seas to try your nerves. The hope was for the first week you would ease into the rough stuff. I'll take 6-10' seas that first week. The motion made me tired. After that first week, throw 25-30' seas and we'll deal. The thing we didn't want to think about was hitting big seas the first week out before you have good sea legs.
Now, in addition to your main job as a DWO or EOIT, and your collateral duties like being a COMMO or PAO, you'll also have weird jobs now and then we lumped into our SLJOs box. SLJO stands for "$hitty Little Jobs Officer". Being the preliminary investigation officer (PIO) for an investigation or planning and MCing a change of command or retirement ceremony would fall under SLJO.
You will fail as a JO. You may fail often. You may start out as a crappy JO. Learn from your mistakes. Learn from and trust the people around you. Let every failure be a lesson for the future and you'll get better. Some jobs you'll love. Some jobs you'll hate. You'll have people working for you whose lives and future jobs depend on you doing the right thing. Praise people in public. Correct them in private. Don't be too hard or too easy on yourself.
Long answer, but I hope it gives you SOME idea, already from the perspective of a first tour Coast Guard JO on a cutter.