Trains depart Glasgow Central High Level. As said, usually on the low number side (Platforms 3 and 4 are a favourite), but be prepared for any platform; I've had one depart from 11A/12 once.
For the best connection to Motherwell, take the xx:43 trains. These allow you to take the next yy:22 trains. At Whifflet, you do not need to change platforms in either direction.
There are also a few services from Glasgow Central Low Level a few times a day in the peaks. These are Class 334/318 operated and run via Hamilton, Motherwell and Whifflet to Coatbridge Central (hence also passing Mossend). If you're there when one of these departs (normally early in the morning or early in the evening peak), these will probably be your quickest option if you want to go past Mossend.
At Mossend itself, I can't tell you where the trains stable; I don't know it too well. But there is always plenty of activity on both sides. I think the locos stable on the East side but I could be wrong.