Save money on the environment and build up skills by repairing things yourself. This is especially so in the case of electronics and electronic devices.
Not everything will be repairable or even possible, with many devices now erring towards non-repair capability and everything being designed not to be opened by the end consumer. But the following is essential equipment for you to cover many repair possibilities.
Opening - Electronics Screwdriver and Bits
This isn't so much a tool, but it's a kit, and I highly recommend getting kits from iFixit. They don't just work with electronic devices but anything with unusual or smallish screws. This is the part of repair about getting access to things inside them.
Diagnosing - Multimeter
A relatively cheap multimeter (although I do recommend going for higher quality brands such as Fluke) is a versatile tool in helping diagnose issues. I'll be honest the most frequent thing I do with mine is checking on batteries in the first instance. You get a much better idea if a battery has lost too much charge.
It's useful when checking things that don't require too much knowledge of the repair, such as whether wires are broken, or a fuse has blown. This can save you time and money in at least ruling out things and less chance of waste replacing things that don't need to be replaced.
Repairing - Soldering Iron
Another tool that's essential for actual repair is a soldering iron. It solves the most common reason for something being "unrepairable" as far as electrical circuits and components, that something has disconnected or got cut, or even a blown component somewhere. It is an acquired skill to become competent in soldering, so it could be worth investing in a DIY electronics kit to practice soldering.