Welcome.
/usr is awfully small.
You will find the
du(1) command helpful in determining which of the major hierarchies in /usr (that are not mounted in their own partitions) are consuming the most space. I will assume that a significant amount of /usr space is consumed by /usr/ports, /usr/xenocara, and /usr/xobj (if you were building xenocara at the time).
You can place these in their own partitions, or, you can reconfigure partitions to make /usr larger. There is a
growfs(8) utility, but that can only be used when there is free space on disk beyond the partition to be resized, which is usually not the case. Based on what you posted in your image, it looks to me that in order to grow /usr (wd0f), you would have to move /usr/X11R6 (wd0g), /usr/local (wd0h), /usr/src (wd0i), and /usr/obj (wd0j). To increase /usr space, you can either back up and restore, or replicate to free space on the drive and use growfs, or just re-install.
I generally recommend that when people are NEW to this OS, that they start with a single, large partition .... until sizing for their unique environment is determined.