New to ML and NLP. Is topic/subject extraction a good place to start? I'm new to AI and specifically NLP, I always love to study things as part of a random project I decide to build, right now I'm working on a chat bot and I was looking for an easy way(in terms of code) to extract the main topic/subject from a sentence(in English), the problem is that wherever I search, even if it's a simple code, it combines a full sentence "meaning"(device parts of speech), I don't like to implement something I don't understand so a small step for how to extract only the subject will be really good and can help my bot with some directions, Is there a simple code(don't care what language it written with) to extract, only main topic/s from a sentence(in English)?.
You could learn about word embeddings. These will provide you a natural path to topic models, and many other NLP tasks. Look up word2vec.

I think it is better not to learn two new subjects (NLP and ML) at once. I would start with ML on its own; take this class perhaps. A good place to start is linear regression, or binary classification.