That is a loaded question, kind of like "what do I need to build a house." The answer is fairly complex, but let me see if I can at least hit the high points.
Check Any Local Regulations
The first thing you need to do is to check with your local city or county planning department to learn what the regulations for the address where you want the sign placed.
What they will tell you first is whether the zoning in that area allows for a sign, and if so, what is the maximum size for that sign, if you are allowed to have an internally illuminated sign or neon sign or electronic readerboard or just a monument type sign, or even just a banner or a 4 foot by 4 foot plywood sign. Not to worry, there's a regulation for everything you do in your business in most 1st World countries (such as the US, Canada, and W.Europe).
Assuming that you can put a sign in the zone you want to place your business in, they will require you to provide some sort of drawing that shows how your sign will be installed.
If you are planning some type of monument sign with metal letters or the like, or an electrical sign mounted on a monument or a pole, it is likely, especially if it is a large electrical sign mounted on a very tall pole 20' or taller, that you'll need to have an engineering firm clear the installation method for wind load and weight and whatever else engineers sign off on for loads of cash.
Once the engineer and/or the city planning department has signed off on their approval that the sign will not fall on their Mom, they'll issue you a sign permit for anywhere from a few dollars (in small towns) to ridiculous sums of money that would make George Washington go bald... oh, wait, he was bald.
Once that is done, often in conjunction with a professional sign company, now you are ready to have the sign put up - unless you are particularly skilled and have the tools to do it yourself.
A simple sign can be installed by a sign installer from your sign company or you can do it yourself if you're trying to save coins. As I said before, you can install most any sign by yourself if you have excavating equipment, experience with building concrete forms (or have watched at least one YouTube video on the subject), a boom truck, and an electrician's license (in many municipalities).
For sake of this discussion, let's say you are having a 12' x 18' sign installed on a metal pole 37' in the air. Let's also say you're having XYZ Company install the sign, a reputable local sign installer.
The first thing they'll do is dig a huge hole. This is for the concrete which will surround the pole. The engineer required enough concrete so that just in case there are 2000 mph winds, this sign will not EVER fall down. Or thereabouts.
Next they'll place the pole in position, plumb and level it, and weld stabilizer bars to it so that when the concrete is poured it will, hopefully, stay exactly where it was placed. Electricity will also be run up inside the pole in most instances when possible.
The next step will be to let the concrete harden, then cut the stabilizer bars off the pole. Now they're ready to place the sign on the pole. This is where a boom truck comes in very handy. The sign is lifted into place, and for larger signs, usually a 2nd truck will have a "bucket" where a professional sign installer will weld the sign into position - hopefully plumb and level.
Lastly, the sign installer will add lamps inside the sign cabinet, hook up the wiring to the sign cabinet, install the sign faces, and paint the pole or install a pole skirt.
So, that is, in a few hundred words, what you need to do (or who you need to hire to do it) to install a sign.
ABOUT THE AUTHOR
Barry Brown has been in the Sign, Banner, Decal and Display Business for over 20 years. It isn't what he thought he'd do with his life, but he says he knows too much now to do anything else!
He has been marketing these products online since 1998, and the company he was general manager of in 1998 was the first sign company to be listed on Yahoo!