Blight. It happens to everyone everywhere on earth because the fungal spores are ubiquitous. You can slow it down by spraying copper fungicide (there are other control methods if you google "how to control blight on tomatoes" but copper fungicide is tried and true and very non-toxic) coating the entire plant top top bottom once every week or two once it gets to mid summer. You could start spraying as soon as you plant but usually blight doesn't appear until at least mid summer, like middle of July. Prune off any branches and leaves with spots on them ASAP, it helps slow the spread too. You may very well end up with only a few small branches left on top but if you don't prune, those would end up dead too.
Just also noting that it COULD be some other fungal leaf spot, there are a handful that nightshade plants get commonly. Fortunately like 99% of the time when it looks like that, it's a fungal disease that can be treated by copper fungicide even if it's not actually the blight (Alternaria) fungus.
