Q: Startup disk full after a month of buying it?

Sometimes reindexing Spotlight can sort this out.

go to system Preferences> Spotlight > Privacy

click the + button and add your SSD.

Quit System Preferences.

Open System Preferences, remove SSD from the Privacy list

Quit System Preferences

Restart Mac

Spotlight should start indexing, once that is complete

see if there is a difference in the space used, under About This Mac.

OS X: What is "other" space in About This Mac? - Apple Support

what is storage "other" and how big should it be?

How can I clean out 'other' space on my mac. Its currently taking up 120GB of space.

OS X: What is "other" space in About This Mac? - Apple Support

121GB is not very much. After you get your operating system and a few apps installed there is not much space left for data. It does not take long to fill up 121GB if you are saving videos, etc.