It looks like the beginning of tomato blight. It's very common, unfortunately, so there's a lot of info on the internet. If you want to get serious, look for something published by your local agricultural extension service (so the address will end with ".edu"). Otherwise you can still harvest some fruit before the plant declines. It all depends how fast the fungus develops in your conditions (e.g. weather, humidity). You can remove the sick leaves and don't splash water on the plant -- water the base only, so the spores don't land on healthy leaves.

Thank you sooo much! I ordered an organic fungicide to treat it and hopefully it will be better. I live in Amsterdam actually and it's really hard to not have water on the leaves of the plants because at times it rains for days. The thing I didn't do and I read it really helps is mulch. I should have mulched. I thought people did it only to keep moisture and I was like "yeah right, like I need to keep moisture when it rains all the time". It has also definitely spread to the aubergines and my pepper, which makes sense. Well now I learned something!

Dora, I also used to live in an area with very hot, humid summers and lots of rain. I had issues with fungal diseases all the time. But somehow I managed to get a decent harvest, including tomatoes. So don't give up!

Mulch helps, as well as removing bottom leaves as the plant gets bigger. The spores overwinter in the soil and land on the bottom leaves with splashing water. So mulching deep helps to isolate them a bit and keep them away from the plant.