Even the best gardeners experience tomato troubles. If you are just starting out, don't let this deter you from growing tomatoes (aka love apples, historically). The benefits are worth it.
Before we get into the details, let me say that you can eliminate a majority of the tomato diseases by rotating your plants every 3-5 years to a new place in your garden. Soil born and fungal diseases live in the soil. Tomatoes (which are part of the nightshade family) create a fungus in the soil which can be harmful to the plant. If you get diseases and viruses one year, and don't plant in a different spot the next year, you will exasperate the problem. Plant a crop that is not in the nightshade family in that spot next. No potatoes, peppers or eggplants. Think beans, greens, brassicas, squash, etc. Practice good garden clean up. Remove as much of the plant as possible and put it in the garbage.
Another practice for healthier tomatoes is related to watering. Water as early in the day as possible. Watering in the late afternoon or evening keeps your plants and garden wet all night; conditions more suitable for diseases. Water your plants from the bottom to avoid getting the leaves wet. Avoid splashing water from the soil to the leaf level. As the soil splashes up to the leaves, the fungal diseases will begin to affect the plant. As stated before, fungus and viruses live in the soil. Fungal diseases come in many forms. The most common ones are early and late blight and leaf spot. Again, the best way to eliminate these is by crop rotation. You can control this with a product, but use an organic fungicide. You can also make your own with a little water, tea tree oil, and dish soap.
In addition to watering, keep the spacing in mind when planting your garden. Two feet is ideal between tomatoes in all directions. This space increases air flow. Tomatoes need a lot of ventilation to keep things as dry as possible.
Finally, as the plant begins to grow and get taller, you can begin to remove the leaves from the bottom of the plant. This slows down the spread of fungal diseases.
"Blossom End Rot" (BER) is not a disease but rather a fruit issue. One day you go out to harvest a tomato and notice the bottom is black and rotten. The main reason for this is a lack of calcium being taken up by the plant. Before you go out and give milk and egg shells to your plant, read this. Most soils have sufficient calcium in them already. The problem is that if you don't have the correct pH in your soil, the plant can't take up the calcium. Get a soil test. Most vegetables grow best between 6.5 and 7.0 pH.
Another way to prevent this is by mulching your tomato plants with compost, shredded leaves, or grass clippings from an untreated lawn. Tomatoes like consistent moisture.
Again, don't let all this keep you from growing tomatoes. There is nothing better than a tomato fresh from your garden! Stay tuned for information on processing your tomatoes.