First take a class on firearm safety or a concealed weapons class which will teach you a lot about the firearms and self defense laws in your state.
Your main problem today is ammo (22LR is particularly hard to find.) Your choice of firearms is also limited. Lots of new first time buyers since all the riots and defunding of the police in the last few months. Your limit of $300 is also a problem in today’s environment. You may consider saving some money or waiting until the demand decreases and look for a used handgun.
My advice is to first buy a .22 long rifle semi-automatic pistol such as a Ruger. Learn how to be proficient with handling a firearm and develop your accuracy skills. 22LR ammo is cheap compared to other pistol ammo and by the time you have developed skill at shooting you will have saved enough money on ammo to buy another handgun. A .22LR handgun also has little recoil which can interfere with your learning curve. While not my first choice by any means a weapon such as the Ruger pictured can be a home defense gun. You have a 10 round magazine in this weapon and one in the chamber if you choose. With practice magazines can be swapped in a second.
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Then much will depend of what you plan to use your weapon for. For home defense there is thing wrong with a revolver. Revolvers are simple to use and reliable. A Smith & Wesson .38/.357 revolver loaded with .38+P ammo such as this S&W Model 66 would be a good choice.
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Many people would rather have a full size pistol such as a Glock 17 9mm for home defense because it holds more ammo. I don’t own one but have fired a number on the range. They are excellent firearms. (I have small hands and the Glock 17 has a wide double stack magazine which makes me slant the firearm to reach the trigger.)
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For concealed carry if you live in the south you might chose a small compact revolver or pistol as they are easier to conceal under light clothing. If you can legally open carry you are not limited except for the fact that a heavy weapon is tiring to carry.
I carry a Smith and Wesson Model 642 .38+P revolver concealed in Florida in a pants pocket. It is an extremely light weapon but is not considered a beginner“s handgun as it has a stiff recoil and is double action only. (You can’t cock the weapon but must pull the heavy trigger.)
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I recently bought a Ruger EC9s 9mm subcompact pistol which I may start carrying concealed. I need to practice with it first.
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Most people do not shoot their weapons often so it is quite possible to find an excellent used weapon to purchase. The problem is the price will be high due to the demand and supply.