Actually, it seems like opinioins are divided about this.
According to my theory books, a trill starts on the upper note except when it is immediately preceeded by the upper note, in which case the trill begins on the lower note (so as not to repeat the same note twice).
This is what my piano teacher told me also. However my organ teacher (who is more steeped in modern but historically informed performance) says this is rubbish and you should start with the upper note even if the preceding note was that same note already. This argument was about Bach trills but it will be the same for Mozart, I guess. Note that this is only for the trillo. A mordant will always start on the main note.
Personally I'd go halfway and say that if you have the time to do it, start on the upper note. But sometimes in faster pieces you just can't get that done, and then better to start on the main note.
Hope that helps !