A picture would be helpful, Lizzy. There are many causes of brown lower leaves: everything from wilting between waterings, to incurable fungus diseases.

how do i send a picture

You got it!
Sorry, Lizzy, it's something that I haven't run across, yet. I have sent a private message to our resident GoY veggie guru, and hopefully she will send better info.

Well it isn't blight, which is good news for you Lizzy. My best 'guess' is a virus and I would simply remove those leaves from the vine. Make sure you have good air circulation around your tomatoes. Don't allow weeds anywhere near your toms. and remove any leaves that are touching the soil. Keep well watered and once the vine is in flower start to feed with a liquid tomato feed once a week. You could also use a garlic spray as a systemic to help to keep beasties at bay. You can either buy this as a concentrate in your local GC or make yourself. Take three complete garlic bulbs and put into a blender with a small amount of water. Blend until you have a thick liquid. Strain, and make up to about a half litre with more water. Store and use diluted 1:10 in a sprat bottle. This will not harm your tomatoes and after the first few minutes you will not smell the garlic but the wee beasties will. Repeat once every ten days or so.