It looks like an attempt was made to render the wall sometime in the past. You can see that parts of that render have fallen off and some have stayed. In its current state, the proper way to render this wall is to remove as much of the paint as possible. I suggest using a wire wheel to key and strip the surface of the concrete.
The paint that is currently on it acts like a surface sealer and prevents the render from integrating with it. If you render it in its current state, it will eventually crack and fall off. Stripping the surface of the concrete will also remove any loose concrete that's just hanging on to the surface of the wall. Once the entire wall has been stripped, I suggest using a pressure washer to clean the wall and prepare it for rendering. Once the wall is totally dry, brush the surface off with a stiff broom to make sure that there are no loose debris left.
The basic tools you'll need are a trowel, float, sponge and large plastic container for mixing the render in. But if you have a wheelbarrow you can mix the render in that as well. The general steps will be to mix the render and applying it to the wall in sections using the trowel. The float is used to give it a level finish and the sponge to finish off the surface.
Please make sure to wear personal protection such as gloves, goggles and an appropriate mask when working on your project. Make sure to follow all the recommended safety instructions at the back of the packaging.
If you need further assistance, please let me know.