Let us be honest here: guns are first and foremost for killing. From small .22 Long Rifle derringers to express rifles, guns exist to propel small pieces of metal to high velocities in specific directions. The second rule of gun safety is to never point a weapon at something you do not intend to destroy. All this being said, a modern semiautomatic carbine is an excellent tool for this purpose. With the recent election, there has been a marked increase of those on the left end of the political spectrum in this country taking advantage of their Second Amendment rights and buying such things. I have been pleased to see this, as I am a firm proponent of an armed society. So allow me to walk you through the selection and purchasing process.
To start with, you should determine what weapon fits your need, and (depending on where you live) what is legal for you to own. I am not a lawyer, and am familiar only with Texas gun laws, so your mileage may vary. A good place to start if you have no experience with firearms is a semiautomatic .22-caliber rifle. The Ruger 10/22, Marlin Model 60 or similar is your best bet. They have large aftermarkets and are highly regarded, especially for novice shooters. Good fundamentals are key to successful shooting. The Ruger 10/22 is an very common model and is widely used as a training firearm. It has a detachable rotary magazine, while the Marlin has a fixed tube-style magazine.
For more strenuous applications, a good combination of rifle and handgun is suggested. A pistol is better suited to indoor work, as the weapon is smaller and more maneuverable in close quarters. Additionally, handguns generate less noise and muzzle flash. A rifle is more useful for outdoor applications, and a modern semi-automatic carbine is as close to an all-around weapon as there is. I personally recommend a Glock 19 handgun and an AR-15 type carbine. Other options are the Colt 1911 and the many copies thereof, or an AK-series rifle. Your mileage may vary.
The Glock 19 is a semiautomatic pistol, chambered in 9mm Parabellum. It is striker-fired, lacking a hammer. It also lacks a manual safety, relying instead on internal safeties to prevent accidental discharges. These pistols are versatile, suited for both concealed-carry defensive roles and for use as a duty weapon. Though perfectly serviceable in the factory configuration, an enormous aftermarket exists for those wishing to customize. I suggest tritium phosphor night sights as a starting point, and a holster is a must when buying any pistol. At the time of writing, a good price is under $550.
The AR-15 is a semiautomatic rifle, most commonly sold in a carbine configuration by a truly huge number of manufacturers and typically chambered in 5.56mm NATO. To attempt to list in detail all available options would be futile. I would recommend buying a rifle from Aero Precision, Smith and Wesson, or Palmetto State Armory. I'm not even going to try and nail down a good price, but it should be less than a thousand dollars.
Having selected what you want to buy, the next step is going to a gun store and handling it. Get a good feel for it, and don't be afraid to ask questions of the store staff, or check youtube for reviews prior to going out. Be sure to know and understand the four rules of gun safety. Before buying it, you should attend a class on basic handling ans gun safety. These are frequently given at local gun ranges, usually for a small fee. For those with children or roommates, a locked safe or cabinet is also in order. Once you've settled on your purchase, you get to buy it. If you're buying from your friendly local gun store, you will have to fill out an ATF Form 4473, which is a standard questionnaire in which you certify you are legally permitted to own a firearm. The store will then call in a background check to the FBI, and (barring any state-mandated waiting periods) when it comes back clean you pays your money and takes your gun. The process is quite similar at gun shows, since most vendors are licensed dealers. Buying from an individual obviates the background check, but can be a hassle. Many people are particular about their pricing, or will only take cash.
Once you have bought your weapon, go to the local gun range and practice. When handling a weapon, treat it as if it is always loaded, no matter what. Never point a gun at something you aren't prepared to destroy. Keep your finger off the trigger until you are ready to shoot. Know your target, and what is behind it. Buy large amounts of ammunition, and take lessons if you feel the need. Be sure to become proficient in the use of your weapon. As always, follow all state and federal laws. There are various internet communities which will offer more or less helpful advice. Guns are tools, which you have a right to own. Exercise that right.