It depends what you want. If you want a mundane gallery style, grab a bunch of paintings you like put it all up in random orders spaced evenly and who cares, you're only in an apartment which means you'll probably move at some point. However, if you want cohesive decor, you want to tie the pieces into the entire room, which means you need to be cognizant of styles, colors, lighting, etc. for the art but also you should tie it into the paint color, moulding, wall sizes and shapes, furniture style and colors, etc.
I would first decide a general plan - theme color palette and size/number. For my main family room this was cool colors (blues/grays - matches the paint and trim), I needed several pieces and various odd sizes/shapes from tiny to massive. As for theme, my house has a big mantle and I couldn't live without a model of the USS Constitution as a mantle piece, so I decided to make it the center of the art selections and build out from there.
This made it pretty easy to decorate, we have a skylight, a pool and an open floor plan so to me I felt some light nautical theme was going to work well because I had my mantle (centerpiece), I wanted to make the room pleasing and not clash with having the back of my house open to the pool and sunlight pouring in.
That said, you don't want to go full Disney theme resort either. Give you an example, I put up an antique German barometer on one wall. Not really nautical at all, but actually functional, a neat piece and tied into the decor aesthetically. I do have one seascape on the wall, which is about as close as I get, but it's just a very calming picture and fits without yelling "hobby lobby aisle six". I also found, randomly, outdoor LED old fashioned looking brass lanterns on a rope that perfectly replace the garland in non xmas season and provide great ambient lighting on the mantle during the summer nights. I'm still looking for one more piece to fit a very long narrow stretch, I think I've settled on a metal wall art with the lacquered finish, something a bit abstract but the same color palette and with the curves to evoke waves without hitting you over the head with it.
So this is just an example of some thought patterns on decorating. Consider things other than "canvas" (paintings). Metal wall art, enameled photo/painting prints with phosphorescent inks, decorative lighting, clocks/weather instruments, models/statutes, shadow boxes (probably not great for apartments...), switch plate covers, tapestries/hangings, light fixtures, curtains, etc., etc.
Just focus on developing a coherent plan first and find your centerpiece for inspiration to build around. Shop retail, antique stores, online, look for stuff that fits the theme, size, palette, price range and the space.