I’ve not yet done much with ngSpice in KiCad, mostly due to focusing and concentration problems, but I would not easily go to using an external simulator.
ngSpice itself is quite mature and decent.
There is a learning curve involved, that much is sure. And the lack of included (and compatible) spice models is quite a big stumbling block for beginners.
I do recommend to start with very simple circuits such as the LED with series resistor posted by BlackCoffee. A blinking LED project is a good way to start with the basics. Whether it’s your first breadboard, your first uC program or a Spice simulation.
If you can get your LED to Blink in ngSpice, then you know you’ve set up your voltage sources and model for the LED properly, and also the simulation parameters. With such a simple circuit as an LED you can also concentrate on whatever that’s needed to setup the simulation, and only later expand on that by adding an opamp, passives or other stuff.
Studying working examples is another resource to exploit.
There are some built in examples in KiCad (if the Demo’s are installed), which among other things have a Sallen-Key filter with an opamp. There are also some examples gathered in:
Search for the book BlackCoffe mentioned: