Don't add refrigerant unless you know why you are adding refrigerant, and do so to a pressure/temperature chart with a good high/low manifold kit. And even that is not exactly considered "correct".
Otherwise, you are rolling the dice. Maybe you solve something, maybe you make things worse. Flip a coin.
Things you absolutely can do as an uneducated consumer?
1. Gently clean the condenser with a mild degreaser. I have found Z-clean to work well without messing up aluminum or paint.
2. Check/change cabin filter
3. Check vent temperatures. If you're getting vent temps about 30°F colder than ambient at idle, that's about as good as it gets. Check the vent temps again while misting the condenser with water. If the vent temps get ridiculously cooler, that's an indicator of a condenser cooling problem.
4. Make sure that if you are at idle, and put AC on max cool, or highest setting with recirculation. Leave all doors open. You should not see the compressor cycling. Should be fully engaged. If it's cycling, that could be an indicator of low charge.
If you really want to get "DIY Serious", get a set of manifold gauges, a vent thermometer, and a laser thermometer. That's enough for one of the many Auto AC educated people here to guide you in the right direction.
Again, I'm not suggesting you do this. I'm only telling you what you want if you really want to DIY this. A good AC shop is the better choice if you just want to optimize your system with the least amount of work and hassle.