This is common as the plant throws its growth energy into the top and fruit production, but usually it is only the lower leaves or minimally sporadic within the plant as summer wears on . It can be the result of inconsistent watering or a fertilizer issue. Tomatoes are heavy feeders and love heat and all the sun you can give them, but they pump a lot of water through their systems; as a result it is one of the most dynamic plants in the garden or on the patio. The brown spots can be fungal spore breakouts on the weakened yellower leaves; the spores can activate from surrounding dirt splashed on the lower leaves from during watering or rain. They are best watered with a drip system or use care in other methods not to splash leaves near ground level. Keep a regular watering schedule.
My potted tomatoes draw water from their pots' bottoms because I use two-piece pots with lower openings where I add water without wetting the top soil; until they were a month or so old and got their roots down, I did water from the top though. Still, I inevetably get yellowing of lower leaves and occasionally others as the summer goes on. It could be an iron deficiency in one soil section, an unusually cool night or two, or any of a number of causes. I don't fret unless it spreads alarmingly - it's blackening stems you really need to panic about, as that disease spread through many of my garden plants during 2009's radically cool, wet summer, ruining tomatoe production. It sounds however like your yellowing is advanced far more than usual.
The more sun the better, 6 hours a day minimum, and limit any shade. Lots of slow-release fertilizer, but not right on the roots, preferably via the watering system, but not too strong. Choose tomato varieties that are disease resistant as per their labels; some are more susceptible to tomato viruses and fungus infections than others. Your yellowing could easily be Early Blight, a common fungus. Aphids that are out of control can devastate a plant also, but it really sounds like Early Blight. Use an all-purpose tomato dust to control both, but start early to get ahead of the problems. Once damaged, the plant has to play catch up all summer.
I also use bloom setting spray which improves fruiting. Fertilizer should have an iron component and not too much nitrogen. Many companies make tomato "food", organic and otherwise, pelleted, stakes, and soluble versions. I vary which I use during the summer. I start with Miracle Gro moisture controlling potting soil in pots and also immediately surrounding garden tomatoes, and I remove some lower stems and bury the seedling to the hilt to develop more roots along the stem. A couple inches under the bottom roots I bury a little bone meal, dried grass clippings (some fibre, haha), and some pelleted fertilizer or a little Alaska Mor-bloom liquid fish fertilizer.
During the summer every couple weeks I alternately scratch in pelleted fertilizer and a sprinkling can of Mor-bloom/water mix, and every 10 days every outdoor plant I have, yard, garden, and patio, gets a dose of soluble from a hose-end sprayer; as the summer advances I graduate from regular stuff to low-nitrogen bloom-encouraging solubles such as Miracle Gro Bloom Booster and Alaska Mor-bloom or one from Rapid-Gro, to milk out maximum flowering.
This is probably more than you wanted to know, but 130 years of experience between my wife and I, from farm to suburban garden, gives us pretty good results. During that very unusual incessantly cloudy and cool 2009 season, all our garden plants fought so desparately to find sunlight that they went for the sky. Out normally 8 ft. sunflowers went to 12 feet, and our cherry tomato hit 10 feet! That was fun trying to keep it corraled so it wouldn't topple. But what a crop! At least until it finally succumbed to the stem-blackening infection I spoke about, from the cool humidity.
Hope there is some help for you somewhere in all this. Good luck.
