まずは少し文法を勉強しないとな ^_^

Which is to say: to understand this construction, you "need to" learn how to express “need to” in Japanese. I recommend you read the following comprehensive *Learn Japanese* article from start to finish: [*Expressing “must” or “have to”*][1].

As you'll see, there are many variants of the same phenomenon/construction, but about 3/4th down that page you'll find this section which matches your sentence:

> The 「と」 conditional is also used by itself to imply 「だめ／いけない／ならない」.
>
    学校に行かないと。
    Gotta go to school.


  [1]: http://guidetojapanese.org/learn/grammar/must