How to Do a Wall Push Up
To do wall push ups, start by standing just over 1 arm's length away from the wall, keeping your feet shoulder-width apart. Next, put the palms of both hands flat against the wall at shoulder height, approximately shoulder-width apart. Then, slowly bend your elbows while keeping them at your sides, and lower your upper body toward the wall as you breathe in. When your chest touches the wall, pause for a moment, and exhale as you push yourself back up. In order to get the most out of your workout, aim for 10-15 repetitions per set.
Find a spot. Before you start doing wall push ups, you should make sure the wall you've chosen can accommodate push ups. An ideal spot should be away from any corners so that you have a long, straight wall. Additionally, the wall you choose should be free from any objects or obstacles. Make sure there is nothing hanging on the wall where you decide to do push ups. This includes picture frames, decorations, etc. Choose a blank part of the wall that is also far from any potential hazards, like windows or kitchen "pass through" window holes (for a breakfast bar). Get your footing and hand placement. Proper stance and positioning is important for doing a wall push up. If you are too close to the wall, you won't get a proper workout and could potentially hurt your back. If you're too far from the wall, you could risk falling to the floor or hurting your back from arching it uncomfortably. Positioning yourself may require you to lean slightly, so make sure you're ready to perform the wall push up once you're in place. While facing the wall, stand just over one arm's length away. For most people, a comfortable stance will probably be between 12 to 18 inches (30 to 45 centimeters) away from the wall. Keep your feet shoulder-width apart. Put the palms of both hands flat against the wall at shoulder height, approximately shoulder-width apart. Lower yourself against the wall. Controlled movements are important in this (or any) form of push up. If you let yourself drop too quickly, you could lose your balance, yet moving too slowly could cause you to get tired out very quickly. Keep your feet planted firmly on the floor. Do not lift or shuffle your feet at all while doing push ups. Slowly bend your elbows while keeping them at your sides and lower your upper body towards the wall (without hitting yourself against it, of course). Lower yourself to a count of four. That is generally a safe span of time in which to drop down during a push up. Breathe in while you are lowering yourself. It is much easier to inhale while you are doing the "easier" portion of an exercise motion and to exhale on the strenuous component. Keep your back and hips straight as you lower yourself. Pause when your chest and/or chin is touching the wall. One or two seconds should be sufficient. Lift yourself back up. Once again, a slow and well-controlled motion is crucial during wall push ups, and that includes when you are lifting yourself back up. Do not hurry or you could hurt your back, shoulders, or elbows. Exhale as you begin to push off the wall and hold your elbows in at your sides. Lift yourself up to a count of two. You shouldn't need as much time as you took lowering yourself since there is not as much risk of hitting the wall while returning to your start position. Continue to keep your feet firmly planted with your back and hips straight throughout the push up. The push up is complete when you have completely returned to your starting position. Make sure that you do not lock your elbows once your arms are fully extended or you could injure yourself. Repeat. By now, you have completed a wall push up with the proper form and positioning. But you will not build any muscle by doing one single push up! It's important to work out in sets with a predetermined number of repetitions per set in order to get the most out of your workout. 10 to 15 push ups is a good number of repetitions in a given set. Rest for one or two minutes after finishing your first set. Then, if you're able to, try to do another set of 10 to 15 push ups. Gauge your progress. Doing wall push ups is a great way to strengthen your pectoral and tricep muscles. However, just because you can do a few sets of wall push ups, it doesn't necessarily mean you should move to the floor just yet. It takes time and consistent effort to build muscle mass. By some accounts, it can take anywhere from six weeks to six months before you build any noticeable muscle mass. Make sure you have adequately built some muscle before trying to advance your workout routine into anything more challenging. Progress to a lower surface. Once you have built some muscle from doing wall push ups and can comfortably do multiple sets, you may want to progress to something more challenging. Before you tackle traditional push ups on the floor, it's a good idea to work your way down to lower and lower levels (and build more muscle mass in the process). First, try doing push ups off of a counter ledge. It's slightly lower than a shoulder-height spot on the wall, but still easy enough to execute the movements without much difficulty. Progress to a couch armrest. The armrest of a couch will be slightly lower still than the counter. Just make sure your couch is sturdy - have someone sit on the opposite end if there is any risk of the couch flipping over on you. You can also try practicing on the first or second step on a set of stairs. Finally, after several weeks of training at each incremental height, you may be ready to try the floor. Doing a regular push up off the floor is difficult though, so make sure you are prepared for it. There is no objective measure of when you are prepared to advance your workout. Only you can decide when wall push ups (and each subsequently lower surface) become easy enough to do comfortably. Consider doing bench presses. Eventually, after many weeks or months of doing different types of push ups, your workout routine may hit a wall. As you get stronger and build up more and more muscle mass, push ups (off of any surface) won't offer you enough of a challenge. When this happens, you may want to consider a weight-lifting regimen like bench presses. Bench presses work many of the same muscles as push ups, but have the added benefit that you can add more weight on once your current routine becomes too comfortable. Remember that it's important to measure your progress and decide for yourself whether or not you are ready and able to advance your training to something more challenging. Have someone you trust who is able to lift weights supervise (or "spot") you while you lift weights. That way, if anything goes wrong or if you can't lift the weights, your friend or gym buddy will be able to help you get the weights back on the rack. 
Find a spot. Get your footing and hand placement. Lower yourself against the wall. Lift yourself back up. Repeat. Gauge your progress. Progress to a lower surface. Consider doing bench presses. 