Tom, you need to repair the damaged plaster before covering with tiles, the main reason is that if there are gaps behind the tiling, then they can pop (come loose), as well as not having as much surface area to stick to.
Any plaster will do, though the trade will generally use ready mix joint cement, which is cheap and easy to use. Once you have filled the holes / skimmed the area, give it a key, so that when dry it is ready to go straight away.
Hope that this helps. Mark
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