Paint tends to reflect the structure and texture of whatever you've put it on. This trait makes it challenging to paint a rough surface in order to make it seem smooth. However, if you have the patience to pay attention to detail and the time to apply multiple coats, you can have some success by at least de-emphasising the rough texture beneath your paint.
Fill in any cracks or holes in your surface using painter's putty. Wipe off any excess with a rag, then allow to dry.
Sand down any rough spots, including the surface of your dried putty, using sandpaper.
Buy matt paint instead of glossy. The less reflective surface of your paint will emphasise texture less. Less vibrant colours, such as beige and grey, also de-emphasise the texture behind them.
Apply a coat of high-coverage primer using long, even strokes. Whenever possible, run just one long line of paint for each time you reload your brush.
Wait for the primer to dry. Sand down any obvious brush stroke lines.
Apply your first coat of paint, using the same brush technique you did with the primer. Allow to dry and sand down any rough areas.
Add additional coats until the surface is smooth enough for your needs. It's unlikely you'll ever get it perfectly smooth, but enough coats will make it look much smoother than it did when you started.