Concrete is the most commonly used man made building supply on Earth. Six billion cubic meters are manufactured a year, in a $35 billion industry. This means there are pretty good odds that you'll eventually do something with, or be around something with concrete.
Weather and climate will affect your concrete. It is not wise to pour your cement in conditions that involve a drop below freezing before the cement cures, as this will prevent the cement from reaching its maximum strength. Likewise, if the humidity is too low, the surface of the cement can crack and produce weaknesses for weather to attack. All concrete will eventually crack to some extent, though this can be minimized by good design and selection of good reinforcement where needed. This does not mean concrete is a bad choice, as concrete structures have lasted thousands of years.
Concrete is used as a fire barrier, but while it doesn't burn, it can be damaged by heat. Once the temperature reaches or exceeds 300 degrees, the concrete is considered unsafe, and this can usually be detected by a pink, light gray or yellow-brown color. Discolored concrete should be replaced. For handyman style use, bagged cement is perfectly reasonable, and for convenience, there are pre-mixed bags with aggregate already mixed, just requiring water. These are very handy for small jobs such as setting a pole into the ground for a doggy chain or clothesline. This is usually too expensive for large jobs.
Pouring concrete, such as a sidewalk, requires several steps. First, figure out how much you need. This is determined by measuring the surface area and thickness to determine how many cubic yards you need, and this will tell you whether you can mix it yourself, or need to have it delivered. Then you build your forms. This probably involves digging, and laying 2"x4" boards to hold the edges of the concrete. 2"x2" boards should be used to hold those in place, pounding them deep enough that the weight of the concrete won't push the boards out of place.
You will almost always want to lay reinforcing material in the bottom, such as metal fencing materials. Ensure that you have everything square and at the proper level, as after you pour, it will be too late. Then ensure that all of the pieces are well secured with nails and clamps. Spray the site with later lightly, and pour. Tamp the concrete down, especially on the edges, and level the concrete.
Once it has set enough to support a 2"x8" board, use one as a straight edge to cut contraction joints. These should be every four to six feet. For sidewalks or narrow areas, this is sufficient, but for patios these should go in both directions. A pair of beveled clapboards can be used. Coat them with motor oil and place a nail into one. Insert them into joints on large concrete structures. As the concrete begins to set, the nail is used to remove one of the boards, and the other is hidden inside, leaving room for the concrete to expand or contract with the weather.
There are a variety of ways to surface the concrete, but remember that you must select one that will allow water to drain off of the concrete, as standing water is a major cause of damage to concrete. After you surface the concrete, it must be wetted down every 12 hours for the next three days, and protected from sunlight while it cures. Visit http://www.tipsonconcrete.com for additional tips.
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