To make the point extra clear: は marks the topic of the sentence. が marks the subject of the sentence and is often used with specific verbs, such as 分かります=To understand. (それが分からない, not ...は分からない） Because of this, the が would stress your 私 in your examples in a way you'd probably not desire. 私は寒いです。= I'm cold. 私が寒いです。= Its is me, not him, not her, but me, that is cold. (implication) Also, は can be translated as 'As concerning me, ...'. You're making yourself a topic. Imagine a situation where a teacher asks: 田中さんはどなたですか。= 'Who is Tanaka?' Here, 私が田中です would sound more natural than 私が田中です。 The first implies that I'M Tanaka, whilst the latter presents it like a random fact. If I were you, I'd use Minna no Nihongo. If you want something a bit more playful, then try learning from 'Let's Learn Japanese', which can be found on Youtube I believe. Good luck!