I've been growing marijuana for a month now and the first leaf is starting to look like it's going to break off.

The small cotyledon leaves are not meant to stay, they usually fall off as the plant ages- no worries growmie!

The plant is very small for a month. Water, soil, temperature and lighting must be right. However, it is completely normal for the lowest leaves to fall off, so there is no need to worry. (As far as I know, I'm also a beginner)