Paint color go neutral, like an off white or light grey, that will let you change up the other colors easily without having to repaint. If painting is too much, that yellow isn't bad at all but you'd want to work with that.

Since you've already got a creamish rug on the way, go with off white curtains as well. Don't match the colors exactly to the rug but keep them in the general family.

A small coffee table would work great as well, a wooden one that matches the paneling color (not the floor).

I would also invest in a nice throw blanket for your sofa. Go for comfort over style, and pick a dark grey color.

Some throw pillows for the sofa would also enhance the room, i'd look for any bold pattern pillows that pick up on that yellow wall color with some grey or off white. Lots of different combinations possible, but try to use dark grey/off white/yellow of the walls when picking some out.

The band posters and decorations do say "I'm young", but that's not necessarily a bad thing if you are indeed young. You should like where ever you live, so if it's band pictures then leave the band pictures. Even if you get rid of the pictures, I like the guitar.

If you were to choose some prints, why not go for some art that represents the bands you like? An example would be having candid or artistic shots from concerts or events where the bands you like have performed. Avoid magazine covers, cd covers, festival posters, etc.

Scenery always works well, but art is kind of when you see it you'll know it's meant to be. Try to bring out that blue from the couch and incorporate the other colors you'll be using (greys, whites, maybe some of that yellow) but make the primary colors dark shades of blue.

Again, never set your sights on exactly what you want because discovering is half the fun. Keep your mind open and focus on the colors you want, not necessarily "this specific thing".