The safest bet for use real wood in your tank is to buy driftwood for aquarium use; many (most?) aquarium shops should sell it and many online places do as well. Malaysian driftwood is popular; it is a very dark brown, relatively heavy, and can easily come in large pieces for large tanks (as well as smaller ones for smaller tanks of course). The downside of Malaysian wood is that, since it is so dark, it tends to leach out tannins more than some other types of woods.
Manzanita wood is another popular choice. It is much lighter in color as a rule, and tends to be smaller and branchier...think of what a set of antlers on a deer looks like, that is how a lot of Manzanita wood is shaped. Manzanita tends to not leach out as many tannins, but because it is a less dense wood often it will actually float (for up to several weeks) before it has absorbed sufficient water to sink.
If you choose to gather your own wood, things are a little more complex. Use only hardwoods, not softwoods. Be absolutely positive that the wood you are collecting has not been exposed to pesticides, insecticides (e.g. mosquito spraying like some municipalities do over the summer), or other chemicals. Remove all the bark, any lichen or moss, and any portions of clearly rotting wood. Then boil the wood thoroughly for at least an hour, preferably several hours.
Store-bought wood already prepared for aquariums does not need to be boiled, though you certainly *can* boil it if you are the paranoid type.
It's also worth mentioning that boiling the wood tends to help leach out the tannins faster.
Once you have your wood that has been either purchased or collected/boiled, then next thing to do is let it soak. The easiest way to do this is to buy those big 5 gallon plastic buckets from a place like Home Depot (usually sold in the paint aisle, since people often use them to mix paints). Fill with water, put in the wood, and let it sit for a few weeks. Look at the water every day and when the water is clearly discolored (yellow/tan), dump it out and fill with new water. At first you will likely be changing the water every day or every other day; hopefully after a week or two most of the initial tannins will be gone and you will find yourself going longer and longer between water changes. If you can go a week without needing to change out the water, you are probably set to put it in your tank. (This isn't a hard & fast rule; how long you wait is determined as much by your level of patience as anything else!)
It's also worth mentioning that you don't technically NEED to do the soaking at all; tannins aren't themselves harmful to your fish. However, they are a form of natural organic acid. This means they will lower the pH of your water, so if you haven't soaked the wood a whole lot and you just plop a lot of it in your tank, it has the potential to lead to a pH crash. Of course tannins also stain the water and make the water look "brown," which in general people don't like as it reduces the visual beauty of the tank. But some tropical fish come from water bodies that have pretty high levels of tannins naturally and so feel more comfortable with some tannins in the water.
Those are the basics of using driftwood. Good luck!