Spackling plaster walls is done the same way as you would for drywall. The easiest way to patch small holes and and cracks is to use spackling or drywall compound. Plaster and drywall are both gypsum based, so the two materials will adhere to one another.
Fill the holes and lightly sand them as needed until you have a smooth surface. You may need to coat them two or three times, depending on the size of the hole.
For additional information, you can see the article 'How To Spackle Drywall'. This article has information on the material to use and how to install it.
The main difference between plaster and drywall is that, drywall has a paper facing on it. Plaster walls are generally three different types of plaster, that are build up to achieve the surface. When your holes or damage get deeper into the plaster you may have to do some extra work. See the article 'Fixing Holes in Plaster Walls', for more information.