and welcome to the forum.
Looks like Early Blight. It is caused by a fungus. It usually appears first on the lower branches from spores that get blown onto them or bounce up during heavy rain or watering. (You should always water tomato plants low to the ground at the base and try to keep the leaves dry. If you can't do this, water early in the day so the leaves can dry.) The earlier you can remove these branches, the better or it will definitely spread. (As my tomato plants grow, I always trim off the lower branches on the bottom 1 1/2 feet of the plant.) Low nitrogen levels can favor early blight. Crowding plants can cut down on air circulation. There are several types of chemical and organic fungicides which can help treat this. Speak with your local garden center or nursery to see what they have on hand. Remove your plants at the end of the season and do not compost them. Also, try not to plant tomatoes in the same space year after year.