This has been covered so many times before, but anyway here's my advice:
Wash down walls using a fungicidal solution (local hardware will have this, if not use a very diluted solution of Jeyes Fluid) and a stiff brush. Wire brush any stubborn areas. Rinse off with clean water; a power hose can be useful if the render is sound. Allow to dry before filling any holes and repairing other defects with a suitable exterior grade filler. Spot prime the repairs. Apply your chosen paint in accordance with the manufacturer's instructions, the first coat may usually be thinned down to aid absorption into the render. A minimum of two coats will be required where a strong colour change is not required. If you have problems with efflorecence bleeding through old paintwork try cleaning back and spot priming with an alakali resisting primer like Dulux Sealapore. The weather at the moment is ideal for external work so make the most of it.