a common upgrade on stratocasters are the pickups. most people prefer a "hotter" pickup to the stock one. this upgrade will give you more distortion and definition, resulting in a "fatter" sound.
The most profound change you can make is to swap the pickups, but it's also the most expensive. Alternatively, you could change components like capacitors for less dramatic tone changes.
The biggest change you can make in your sound is from new pickups. You can buy them from most music stores and install them yourself if you can solder.
Most people change pickups - but one cheap upgrade is to buy a Superstrat switch (about $20 from Fender) and try Superstrat wiring (takes work and soldering).
