What is the best starter adjustable power supply for pretty cheap (preferably ac/dc)
First off, I’m in the same boat as you (just 13 myself). By “adjustable power supply” do you mean adjustable voltage, current, or both?

If you want adjustable voltage, you can easily make one using something like an LM317, which is an adjustable voltage regulator.

If you want adjustable current, you can make your own using something as simple as a potentiometer, just check the power dissipation rating.

Building your own is probably the cheapest option, if possible. It’s also fun and a great learning experience (I’ve made an adjustable voltage supply). However, if you need adjustable voltage and current, that can get a bit trickier. If you know how to make one, go for it, but otherwise, here’s a $40 one. Not dirt-cheap, but it’s high quality, will last a while, and the holidays are coming up. Good luck!