Thanks for the pictures, they tell the whole story. It appears that you have a concrete foundation that looked a little bad, so they covered it with plaster and stamped the grout looking lines in the plaster.
An electric chipping hammer with a wide chisel will strip the plaster off. You will find raised lines where there were joints in the concrete forms and I can see, down at the bottom especially, the concrete was not vibrated sufficiently which leaves small holes or indentations.
This can be taken care of with a concrete rubbing stone which will cut the raised lines off an scarify the concrete to take a new finish. Use a whitewash brush to sprinkle the wall and keep it damp, use a circular motion on the stone. To fill the voids, you will need a mix of 2 parts fine sand and 1 part of Portland Cement with water enough to make the mix make a ball in your hand and fall apart easily. You will be adding more water to the wall as you work it in. Sprinkle the wall again and add the mix with a damp sponge trowell. Places that require a thick layer of the mix will need to be sprinkled and sponge floated again at intervals to keep them from shrinking and leaving cracks.
If you want a pattern on the wall you can rub a thin coat of the mix on the wall with the sponge and use the whitewash brush to make a fan pattern or whatever pattern you like. Its kind of like doing a broom finish on a sidewalk. You certainly have my sympathy on this project; it will be a tedious process and a lot of work.