What is the best clothings to wear during 6/12/2014 till 12/12/2014 in Nanning?

Well, the temperature of that period of time will be 28.4 °F to 57.2 °F. It mainly contains sunny and cloudy days.
But you should bring your overcoat, trousers and sweater in order to adapt the big temperature difference. :)

Hi! Lowerest is 28.4 F that is -2 celsius ???

Can you please help to draft a 6 days 5 nites itinerary inclusive of Detian Waterfall. Is it still ok to visit Detian during 6/12 till 12/12, not to cold ?

Do you have good and reliable contacts for local tour guide to arrange ground package?

First of all, I would tell you that the best timing to see the waterfall is during July and Nov. In Dec. there won't be much water, so the scenery of the waterfall is not that attractive.

As for the general itinerary, here is my suggestion:
Day 1: visit the NN city
Day 2: go to Beihai for fun
Day 3: Tongling Canyon
Day 4: head for the Detian Waterfall
Day 5: Weizhou Island
Day 6: back to NN city

I don't know a tour guide. But if you will stay in a hotel, you can ask your hotel to help to arrange a tour for you or ask them to contact a local travel agency.