The topic particle は
The particle は signifies that the word or phrase before it is the topic of the sentence. Normally は is pronounced as ha, but when used as a topic particle it is pronounced as wa. In the first example sentence, は marks 私 as the topic of the sentence.
There are different ways to write Japanese using the latin alphabet, called romanizations. In the west, the most commonly used romanization is the Hepburn romanization, which would romanize 私 as watashi instead of watasi.
We recommend using a romanization that is closer to the Japanese Nihon-shiki romanization system. The reason for this is that it is more regular and has a one-to-one relation to the kana systems. For example, using Hepburn, じ and ぢ are both romanized as ji, whereas with Nihon-shiki they are romanized as zi and di respectively, which makes it possible to differentiate between the two while romanized.
No conjugation for persons
Note that です does not conjugate based on the subject. Whether we talk about I, you, he or she, です remains です.