Ok now, expanding is necessary in all games, even if you are winning. It will help to give you that supreme control that makes the game easy and fun. Well not only do you take a second source of income, but you also take an expansion away from your opponent.
Now with the Terrans, it would be easier to expand to a base that is very close to your main base. Basically you want to extend your choke point, and build your expansion there. Now you shouldn't wait till the last minute. Try taking control of the area by building a bunker and use tanks and marines to hold off. You should do this by the around the time you acquire tanks, since it becomes very easy to defend choke points with the combination of tanks and bunkers.
Don't be satisfied with just two bases. It takes a while to get used to being greedy, but that's part of the game. Be greedy, take any more bases next to yours by using a combination of 2-3 SCVs, 8-12 marines, and 1-2 tanks. You want to close off any entrance, and build two bunkers backed up with tanks before starting on a new base. You can start one at the same time, but I don't advise building one at the new expansion, try floating it from behind your defenses. You want to take as much territory up as possible, so building this new expansion should be as close as possible to your opponents main base. This is to prevent him from advancing closer to you, and makes it easier to siege his base in the future.
Now to take future expansions are harder than your first expansion. Prefferably use the same tactic, except bring more units as goliaths and tanks. You will want to scout any area before you make your long trek to the new expansion, so use a vulture or wraith to scout. You should also bring at least 3-4 SCVs, to set up a defense faster. But try not to shy away from your opponent by building it the corner, you want to seal him off from anymore bases. So take any islands, which should be easy to defend with wraiths and turret. After you sealed off one side, try to expand to any other side that your opponent can still expand freely.
Once you manage to get your expansion, you must build another barracks and factory at the new base, just to produce goliaths, tanks, and marines which are cheap and effective anti-air units. Make sure you get enough turrets around to push any scouting units away, especially queens, observers, dark templars, and wraiths. But just use turrets for detectors as they should be. Now after thinking about it, if the other race is Protoss, we all know that they will build dark templars, so send out SCVs to turret the new base first, instead of building bunkers. Lurkers are also a problem, yet comstats and tanks can stop that.
Now if you are able to hold onto the three bases, you should be able to produce a lot of goliaths and battle cruisers, making any other base easy to take or takout. If you do a good job of containing your opponent, you can easily take any bases that you have behind your front lines. As to finishing your opponent off, try using masses of tanks and goliaths, and use a tank push.
