A base has citizens in it, which can be used to work any tile within range of that base. Tiles can have minerals, energy, or nutrients on them. You can also work tiles by use of a supply crawler.
When selecting a base you should determine what resources are there you wish to harvest with it, as well as if you can defend it.
Rocky terrain can offer you greater minerals, but is less productive in nutrients even if you build a farm there.
Some tiles will have a symbol on them showing a bonus in minerals or energy.
See Resources (SMAC) for information about the resources, and what affects gathering them.
Once planted, forest spread to other tiles on their own.
Planting fungus helps slow down enemies who can not move through it. If you have the research done to have the ability to move through it like roads, its useful to have, and will spread on its own. Also, if you plan on capturing mind worms to use as your primary military unit, its good to have the fungus around for them to move on, they only needing a third of a movement point for every tile of it.
Each faction gains advantages or disadvantages according to what settings you choose. See Social Engineering (SMAC) for more information.