After liposuction Do i have to wear the compression garment directly on skin or should i use it on top of my undershirt?

Anything that is worn against the skin and under the compression garment should be very form fitting. You don't want something bunching or wrinkling under the garment. I would ask your surgeon about this and show him/her how it fits. Good luck!

You can use it
either way. I would recommend using it in the manner that is most comfortable
for you and what your surgeon recommends.

Compression is always encouraged after liposuction to reduce swelling, prevent bruising and fluid collections. Compression is compression so I don't believe it matters what you have on under the garment as long as it fits correctly. 

You can wear something under your garment as long as it doesn't wrinkle or bunch up.  Sometimes this helps at areas like the underarms...if you wear, say an underarmour shirt with sleeves and then put on a compression vest the underarms can be protected from chaffing.As far as panties or briefs are concerned they can be worn over the outside of the garment for hygiene and comfort if you like, as the crotch is cut out of the compression garment.

Your compression garment should be directly on your skin to minimize any irregularities that maybe caused by your undergarments.