OK, you've just bought a super-duper rifle just in time for deer season and you're dreaming of getting the big buck everyone else has been bragging about for years (true or not!). But you can't just buy a rifle and start hunting. It's doesn't work that way. Every single rifle must be sighted in for the individual using it. There is no "one size fits all."
Guide, and 30 year deer hunter, Phil Schweik has provided some excellent info on how to sight in a rifle. Everybody holds a rifle differently. Everybody is built differently... length of one's arms, distance between eyes are examples, and then of course every scope or open sight is unique. Someone else might sight a rifle in, and it will work perfectly for that person, but it won't work for you.
If your rifle has been sighted in by the most experienced professional, but when you fire it you wonder why you're missing everything, it's because the rifle was sighted in for that particular individual. Optical perception can cause you to miss 6 to 8 inches high or wide at 100 yards. It can and does vary that much.
The best place to sight in a rifle is a licensed gun range, public or private. And it's not a bad idea to take an experienced marksman/woman with you to help you, particularly if you're a neophyte. You can also be a do-it-yourselfer. If you have access to a large, safe, open area, you can put up your own homemade target and do it that way. Phil Schweik strongly suggests using a licensed range. Often these ranges will have experts who, if you didn't bring someone with you, will assist in sighting in your firearm.
When you buy a rifle from a dealer, that dealer will usually bore sight the rifle. It's done with a small tool they hook up to your barrel, line it up with your sights, and use a red laser beam to adjust it so that it lines up with your scope cross hairs or open sights and a target bulls eye. This method will definitely get you real close to where you want to be when shooting. So when you do go out to a range to do your final sighting in, you've got a starting point.
One quick point to make the further you get away from a target, the easier it is to miss it. It seems simple but lots of people don't realize this. You may be off, let's say, an inch or two at 25 yards, which translates into eight inches to a foot at a hundred yards. So start your sighting in at 25 yards. Once you've got that fine tuned, move to 50 yards. Then further, as needed for your particular hunting requirements. Most deer hunters will end up sighting in at 100 yards, unless you're heading out West or Canada etc., where you may need to go as far as 300 yards or more.
It is critical that when sighting in your rifle, you always use the same bullet, same manufacturer, same brand, same exact weight, same "point" (such as a "ballistic" versus a "soft point" and so forth). All bullets fly differently. Indeed, sharpshooters will often hand load their own shells. However, most well known commercial bullet manufacturers put out excellent, accurate products.
A few other suggestions: When sighting in/firing, make absolutely certain you have capable hearing protection. That's a must. You can get permanent hearing damage if you don't.
When a woman or kid is sighting in for the first time or so, make sure you start them out with a smaller caliber rifle. Don't give them a 45/70 with a 300 grain bullet or a 7mm mag. Try a .22 or other small caliber rifle. Once they get the feel, they won't be scared away because of the recoil and noise of using a big caliber right-off-the-bat.
There are other peripherals that are available for purchase or use at a range. They include things like a gun vise. A gun vise is a sturdy padded device, often weighted down by lead. Your gun is locked into place and this will eliminate the flinch when firing. It's for the person who really wants to be pinpoint in results, but not everyone uses them.
A number of municipalities have law enforcement gun ranges that they open to the public for sighting in activities. If you know an experienced law enforcement officer it's not a bad idea to offer dinner or a few bucks to have the officer assist in sighting in your rifle. This can be also applied to one of our heroic armed service veterans who probably know more about rifles and sighting them in than most anyone.
Bottom-line: If you sight in properly, you'll be able to place a three or four shot pattern in a bullseye the size of a quarter at a hundred yards. And then of course after the season when you've gotten that 30 pointer, you won't mind buying the next round of beers at the local pub after showing the 3,000 photos you took.