I have plaster walls in my building.
I use a base coat plaster and drywall joint compound to repair them. Depending on how badly damaged they are, you can most often just get away with using drywall joint compound to repair surface dents and such.
You should be aware that there are different kinds of drywall joint compounds, and some will have more glue added to them than others. Joint compounds that are called "Regular", or "Taping" will have a lot of glue added to them so they stick better, but also dry much harder and therefore are more difficult to sand smooth. Joint compounds called "Finish" or "Topping" have the least glue added to them and sand smooth the easiest. "All Purpose" joint compounds are half way between the first two, and are a good choice if you only want to buy one product and use it for everything.
Where the damage to the plaster walls is deeper than just the white "gauging" coat on the surface of the plaster, you might need to use a base coat plaster. These will have powdered glue added to them to make them stick better, and they'll also most often have crushed Perlite in them to give them a lot of bulk without a lot of weight.
Probably the single biggest thing that will improve the results of the plastering work you do is to work with a bright light next to the wall or ceiling to exagerate the roughness of the surface you're working on. If you do your repairs under such critical lighting, then once the repair looks OK to you when viewed under critical lighting, it will look perfect when viewed under normal lighting conditions.
If you do decide to skim coat, here's an easy way for a rank amateur to achieve a uniformly thin coat of plaster over a wall or ceiling:
1. Use a "V" notched adhesive trowel to spread a uniform amount of joint compound onto your wall.
2. Mist that joint compound with a spray bottle to keep it wet and flatten the "V" shaped trowel ridges of joint compound down with a regular plastering trowel until reasonably smooth.
3. Allow to dry and lightly sand to improve smoothness.
Basically, you do a few square feet of wall surface at a time this way until you do the whole wall.
But, repairing plaster is not very difficult, and if you're house has plaster walls, it might be wise to learn how to repair them properly right from the start. Where I live, no one uses real lime putty based plaster any more to repair plaster walls. Nowadays, everyone just uses a base coat plaster (as described above) with perlite in it and drywall joint compounds. I've tried using real lime based plaster (with and without sand in it) to repair my plaster walls, but I found I could get superior results using a modern base coat plaster and drywall joint compounds.