Ok, you will need :
The hardest part about creating a water feature, is knowing what colour to paint it. This really stumped me when I first started out - as I could never get it to look right. Then I realised something - it does not HAVE to look right.. it is water - and when you have a look around the real world, there is no perfectly coloured lake, or stream !
So, map out the area you want - remember to make it irregular - not perfectly round or square (unless you deliberately want it that way !)
Paint the whole area black - yes, black. Let dry. A note here about acrylic paints - you MUST let them dry completely, or you'll stuff up when they get wet.
Now depending on what environment you are trying to create - you may want some DARK blue or DARK green and blend them into the black. This is simple to do - and some people call it 'dry brushing'. Basically, you put some paint on a bit of cardboard and then dip your brush into it, then wipe as much paint OFF the brush as you can, then 'stroke' over the black. you will see light strokes - and it works well. Practise on some spare cardboard - it is pretty easy though.
Ok, so know you have the colour you want - grab your 'secret ingredient' - the Gloss medium. This is available at any art supply place - or online - just google it.
Paint on your gloss medium - you don't have to be neat , in fact the sloppier the better (as it simulates 'waves', or ripples). Once covered - let dry. Come back next day and check . If you are happy with the depth 'perception' you're done - if not - give another coat of medium and again next day until you are. I find about three coats works pretty well.
A tip here, that you can actually MIX a LITTLE paint when you do your gloss medium to colour it a little or add swirls - give it a go - it's EASY and you'll SAVE a heap on commercial 'water' !