Learn Arabic (Modern Standard): لست متأكدا.

The verb ليس (to not be) changes its stem when adding certain endings. For 'I', the ending is ـتُ (-tu). When we attach this, the middle letter ي (ya) drops out to make pronunciation easier, giving us لست (lastu).

Why does متأكدا have an extra alif at the end?

In Arabic, the descriptive word that comes after ليس (the predicate) must be in the accusative case. For masculine words like متأكد (sure), we show this grammatical state by adding an alif with two marks on top, making it متأكدا (muta'akkidan).

If a woman is speaking, how does this sentence change?

The word لست stays exactly the same, because it means 'I am not' for both men and women. However, the word for 'sure' must become feminine: متأكدة (muta'akkidatan). Notice that we do not add an extra alif for the accusative case here, because words ending in a tied-taa (ة) just take the two top marks without the alif.

Do I need to put the word أنا (I) at the beginning of the sentence?

You do not have to! The ending on لست already means 'I'. You could say أنا لست متأكدا, but dropping the standalone pronoun is very common in Arabic and sounds perfectly natural.