2001 Ford KA sound system upgrade?

I've just bought a ford KA and want to get a sound upgrade for it, although I know nothing about cars. I don't want to get a subwoofer but I don't know what my other options are. I've been looking at stereos but will this actually improve the sound or is it just there to look cool? Any help/ suggestions appreciated

Talk to a car audio specialist. Remember you have to tell your insurance company.
An audio upgrade increases the likely hood of the car being broken into for the head unit.

Factory speakers are usually pretty low quality, so replace those first. It's the best and easiest upgrade option. You should double-check, but I believe the door speakers are 5"x7", although can get adapter plates to fit 5.25" and 6.5" speakers instead.

Unless you are prepared to install an amplifier, don't go overboard with high-powered speakers because car radio / CD players don't have a lot of power and you'll be wasting money unnecessarily. Stick with speakers rated 35 to 50 watts rms each. If you want bluetooth or aux input capability, replace the stereo. If you want a loud system, then get a small amplifier and purchase speakers rated in the 50-100 watt rms range. When evaluating equipment, be sure to look at average or nominal power (expressed in watts rms), not peak power output, which is mostly irrelevant.

Also stick with name brands such as Pioneer, Alpine, JVC, JBL, Kenwood, Infinity, Cadence, Kicker, Orion, Polk, Audison, Focal, JL Audio, Hertz, Rockford-Fosgate, etc. Avoid copycat names that sound similar, and brands such as Boss, Crunch, Craig, Kraco, Kenford, Rockwood, and Planet Audio. Price can be somewhat of a guide to quality.