The adverb cегодня is derived, like many other Russian adverbs, from a noun phrase, cего дня ("[on] this day"). Both words in this noun phrase are in the Genetive case, the first one having a typical pronominal declension endinhg -его, in which г is written, but в is pronounced:
сего - pronounced as [си'во],
его - pronounced as [йи'во]
синего - pronounced as ['синива]
(the same rule applies to the variant of this ending, -ого - большого - pronounced as [баль'шовa])
The pronunciation of the letter e as [и] in these words is due to the rule of the reduction of unstressed vowels. According to this rule, the letter e in the syllable that precedes the stressed syllable is pronounced as [и].
So, the IPA for cегодня is [sʲɪˈvodʲnʲə], practical Russian transcription is [сиˈводьня].
Also, you can listen to the recording of this word pronounced by a native speaker of Russian.
Pronunciation with [г]/[g] is incorrect.