Please post a picture of the area under the sink, so that we can see where the pipes go, and what is on either side.
Until we see exactly what you have to work with, it is hard to offer suggestions on what realistically could be done to add shelving.
Hard for shelving under sink with all the plumbing. Also consider access in case of a leak, broken pipes, or replacing faucet or plumbing.
It's really hard to build a shelf under the sink because of the plumbing. You have to have access to the pipes in case of a problem. The best thing is to get storage containers and shelving that is removable and made for under the sink. Maybe something like this would help.
I would cut pieces the width of your shelf to the height you want the shelf to make support for your shelf. Stand these on the sides and tack them to hold in place. You can even use two sided tape.
Then place the shelf on the supports and there you have it. Unless you make the shelf lower than the pipes you will have to make cut outs for the pipes etc.
With this if you need to get to the pipes you can simply pull the shelf out.
Easier still is the curtain rod across the cabinet trick to hang your bottles that have sprayers.