[rfuerst911sc]

A couple of things pop out to me, let me preface I am NOT a pro just a regular DIYer. The large pipe on the left of your pic is for the toilet. The pipe in the floor on the right in your pic is probably for a shower drain. The capped pipe on the wall should be a vanity drain but it's in a weird location in respect to the toilet. It's too close in my opinion but the angle of the pic could be deceiving. And I see no water supply lines.
In my opinion you start with what size shower do you want, and what size vanity. Lay out a pattern on the floor/walls to make sure everything fits and is pleasing to the eye. That is not a huge space so it's easy to over power the space. Once you have those two items layed out and happy with them the toilet is next. You may find that you have to go with a round bowl vs. an elongated bowl as an example. Once the toilet is picked NOW you can start with running your water supply lines AND electrical. I'm assuming you want at least one outlet for hair dryer, a wall light over the vanity and a exhaust fan. You also have to figure out do you want just a mirror or do you want a medicine cabinet ? If you want a medicine cabinet do you want flush mount ? If so now is the time to frame for it. And don't forget about insulation, very important to make it user friendly year round. What about HVAC ? Do you want to heat/AC the space ? If so make provisions for it while framing is open.
There is also a possibility you may want to raise the floor with lumber ( 2 x 4 or 2 x 6 ) this would mainly be for ease of running drain lines IF your layout say's nothing is lining up. Take into consideration ceiling height. Hope some of this helps and good luck with the project.