Here in US, plaster is compound for sheetrock or gypsumboard or drywall.
If you have that here is good video:
http://www.prest-on.com/pages/video.asp?video=repair
In either case, you have to scape all unevenness, use scraper and go as deep and wide as it is required, to firm and stable surface. Than apply plaster whatever it is. If you have mortar/lime plaster and it is significant in size than you need mason guy. After that you need to prime surface and couple of coat of paint.
Hope this help!