How do you decide where on the wall to place shelves?

Its where ever you want them. If its a wall that is blank then measure wall. Mark middle & then you can go full length of wall or do you want empty space at each end? Then you measure how long. Going to hang adjustable brackets or just brackets for each shelf. Use mollies for holler walls & good screws if going into studs. Good luck.

Depending on the wall and what the shelves will be used for - you hang them at different heights. For example, if for a TV - lower, vs. just for storage.

Depends on what you are putting on them and personal preference. Usually eye height for the shortest adult in the household, depending on what is going on it. If you are a visual person, I recommend putting a line of tape in the spot you are considering, stand back as far as you would be able to if there was a real shelf, and see if it looks right. If not, raise or lower the tape accordingly.

how tall are the people in your house? Depending on how far out your shelves will be? You may want to keep the shelves close to how tall everyone is!

Be sure to adjust height if someone will be sitting on that ottoman or bending over above it..not fun to raise up into a shelf, even an empty one..

Good answers, also try to choose a spot that has studs so they are extra sturdy. I agree with DW, I would use her suggestion as a starting point, and evenly space them from there up.

It does depend on the purpose for installing the shelves. I install shelves over doorways to display china. When the china closet original to this house had to be destroyed, I kept the shelves and built a unit around them so I would not lose the history of our home. The unit built fills one whole corner of the room, t.v. is kept here, photos are stored in metal boxes and other shelf space is used for display. The supports for the shelves were re-used and did not need strong wall support as they attach in many places.