Konnichiha, watashi wa atarashii na doshii ga kyo naraimashita!
Very close. Just a few things to point out.
あたらしい (atarashii) is an い (i) adjective so you don't need な (na) after it.
The word for verb is どうし (doushi).
The particle for direct objects is を (wo) not が (ga) (in most cases, as is it for this one)
Today, きょう (kyou), has a long vowel at the end.
Word order is fairly flexible, but you tend to see time words either directly before or after the topic / subject of the sentence.
Also, exclamation points aren't that common in Japanese, you can use sentence-final よ (yo) to serve as emphasis.
I'd personally change it like so:
konnichiwa, watashi wa kyou atarashii doushi wo naraimashita yo.
こんにちは、私は今日新しい動詞を習いましたよ。