You will get a ton of advice about brand and type. The specifics of your use case will have a great deal of impact on the optimal choice.
Short answer, I would recommend training. Then likely a semi-auto 9mm handgun or AR15 rifle depending on the peticulars. Renting or borrowing a few examples and gaining your own perspective is highly recommended.
Most major manufacturers make reasonably reliable products these days. Additionally, any defensive firearm should be trained with, to at least some degree, to validate it operates properly.
For someone of smaller stature, especially without much training, I would avoid shotguns. This is due to relatively high recoil, low ammunition capacity, and a greater training requirement to operate a pump action reliably under duress. Semi-auto shotguns are available, but can be picky about ammunition choice and still have the other drawbacks.
Semi-automatic handguns, specifically in 9mm offer low recoil and simple operation. They can require training to use accurately, but are easier to manuver with if you need to move from one end of the house to the other. I.E. to retrieve children, etc. As they can be used one handed, they allow use of a phone to summon help while still at the ready.
Semi-auto rifles like the AR15 can be easier to use accurately, and carry significant ammunition with relatively low recoil. They are more difficult to manuver with and require two hands, so may be the best option if the strategy is to stay put in an area like a bedroom.
This is just one plebs opinion, and I am by no means a professional in this space, but hopefully it helps.
