Summarize the following:
Before looking for seeds, consult the US Department of Agriculture’s Plant Hardiness Zone Map or your country’s own equivalent guide. Different types of tomato grow best in different climates, so be aware of your region’s minimum and maximum temperatures, as well as how long individual seasons last. Cherry tomatoes may be tasty, but they won’t yield the big fruit you’re looking for. Look for tomatoes listed as Big Boys, Beefsteaks, Colossals, Abraham Lincolns, or Beefmaster Hybrids, among others. Local shops will often sell specific named strains, so if you’re having trouble finding a large species, ask the store attendants. Regional heirloom seeds grow types of tomatoes specific to a single area. Because they have been cultivated in local farms over a long period of time, heirloom seeds are particularly good at dealing with the regional climate. Due to their growing conditions, these seeds are typically sold by specialty shops. Some good, large strains include:  Early Boy tomatoes, a Big Boy variant designed for colder weather and shorter growing seasons. Creole tomatoes, large, slow-growing tomatoes designed or warm, southern climates. Mortgage Lifter tomatoes, a heavy Beefsteak variant designed for long seasons. To make sure your garden soil contains all the nutrients necessary to grow large tomatoes, place a small amount into a plastic bag and take it to a local garden center. The center will be able to test your soil for component pH balance and direct you to any manure or dirt supplements necessary for growing your chosen seeds.

Summary:
Figure out your climate region. Look for large tomato varieties. Look for regional heirloom seeds. Test your soil at a local garden center.