"I will take a message now."
Translation:Gwna i gymryd neges rŵan.
Because 'I will be' is bydda i. byddi di - you (informal singular) will be.
Also, 'n is required after bydda i before a verb noun, and you have a typo in cymryd (no mutation in that pattern):
- Bydda i'n cymryd neges nawr (no mutation of cymryd)
- Gwna i gymryd neges nawr (with a soft mutation of cymryd because it is the object of the verb gwna)
In general use they are equivalent. The use of the simple past and the future of gwneud to form the past and future of other verbs is very common all over Wales in the spoken language.
(Some people say that using gwneud like that is a feature of the language in 'north' Wales, but that is only true if the 'north' starts at Cardiff Bay!)