First upgrade should be a good set of brake pads. Next, I'd look at compressionless brake housing. If your current set-up has a power modulator, swapping it for a standard v-brake noodle will help, or in many cases, you can take apart a noodle that has a modulator, remove the spring, reassemble, and then it works like a normal v-brake noodle.

Upgrading the brake levers could be worth looking into, especially if your bike came with a set of levers that have a rubberized section on the lever blade, those ones can feel spongy and crappy even when hooked up to a high-end brake with compressionless housing. Tektro RS360A would be my pick for a set of cheap levers.