Choose the best
Go for firm tomatoes with wrinkle-free skins and a noticeable tomato smell. Tomatoes have the best, sweetest flavour if they've been allowed to ripen on the vine before they've been picked but, if you buy underripe tomatoes, you can redden them by keeping them in a brown paper bag at room temperature, or on a windowsill.
The sooner you eat a ripe tomato after it's been picked, the better it will taste, so try to seek out locally grown tomatoes if possible. The leafy tops are a good sign of freshness; they should be perky, rather than wilted. Avoid any tomatoes that show signs of mould.
The type of tomato you buy depends on what you intend to do with it. Here's a run-down of some of the most common types.