lucky for you i happen to know quite a bit about concrete.
mix mortar with some sand or fine gravel. make sure its a uniform mix. dig down into the corners of whatever you're mixing it in (i would suggest a wheelbarrow it makes it easy to transport the mix, which is very heavy) and make sure there arent any pockets of sand in the corners. use some type of tool to mix it, you dont really want it on your hands too much. it dries in the cracks of your skin and irritates the hell out of it. a few spots wont do anything you're ging to get it on your hands there no escaping that but you dont want to shove your hands in it.
after you have a uniform 'dry mix' add a little water and mix it in until the mix appears dry. add a little more and mix it again until its dry. keep doing that add water stir until dry (or damp) until you have a uniformly moist but not 'wet' mix. there is a very fine line between good workable concrete mix and runny slop. you'll see, one too many little squirts with the hose and you'll have concrete soup. this characteristic of mortar makes it more difficult to make a good mix with a small amount of it, but since concrete is heavy and hard to work with, a smaller amout is less physically demanding to mix. go for the biggest mix you can manage in the wheelbarrow. putsome of the bags in there and think about how much it'll weigh when its wet.
keep it moist while you work, too. once it dries its no good. a small spritz and a few turns every so often is fine because it dries on the surface first.
you're gonna need a really tight, sticky, almost dry mix for sculpting rockiness on a vertical wall. you wont get that if you've never worked with concrete before. you're going to make soup of it, everyone does. i did. after a few tries though, you'll get it, it jsut takes a few tries to get used to it. you'll see, you'll need less water than you think. remember: the more uniformly mixed the better i cant stress that enough.
even if its not 'soup' but just wet enough that your hand will get wet if you touch the mix it wont work for a vertical wall. it wont stay up. probably the easiest way for a beginner to do what you want to do is to construct the wall on he ground flat and wait for it to hardnen then stand it up.
as far as what to mix and how much, i have no idea. i know all this stuff because i used to work for a mason and they had me mixing it up in wheelbarrows with a shovel. they'd say like, 8 shovels of sand 4 shovels of mortar and 2 shovels of portland or some crap like that, and i'd have to mix that suff that way all day. man i dont miss THAT job.
anyways, i have no idea what type of mix you should be using. you're gonna have to do some research to find that out. or if you know a mason ask them. i dont really remember any specific mixes we did but they were different for different applications. some are stickier than others but what you should be researching is how it will effect the water conditions (i suppose to get around this you could seal it with something non-toxic) and how it will react to being underwater all the time. you dont want it to crumble or crack.
how big is this going to be?