Depending on the reason you want to paint your rendered walls it is always a good idea to firstly brush it down with a broom and remove and loose particles. If you are looking to keep to keep the current colour on the render then you can simply seal the render with a water seal silicone based product like Thomsons Water Sealer.
If you are looking to paint the render then you have a number of options such as Dulux Weathershield and Sandtex. Hower there are other brands you could use such as Crown Stronghold or Johnstones. If you have a modern render such as K Rend or Weber then it might be an idea to check with the product manufacture on what paint they recommend, the reason being that some rendering suppliers also provide their own branded paints.
A good time to paint is always in the summer however if you are looking to paint in the winter then it might be an idea to use Pliolite as that will become waterproof in about 25 minutes. For the best finish it is always an idea to use a roller.