There are several main aspects of self-defense that you should look for in any training:
Basic abilities
One point to keep in mind when deciding what to learn is whether you want to spend a lot of time learning complex martial arts or necessary survival skills. There is a difference.
Many martial arts focus on sports, and while it is still a prominent feature to be eligible in one of them, it may not be the best option in a real survival situation. That is because the movements and skills practiced are for an environment in which there are rules of engagement. So if you later face someone who doesn't follow any rules, this dig can work against you. It can be beneficial to know how to kick a man on the knees or put the finger on his nose.
Mentality
The essential predicate that perfectly applies to self-defense is that the price of freedom is constant vigilance and a continuous willingness to respond.
The crucial aspect of any self-defense training is the mindset, starting at this price of freedom. The ability to remain calm enough to perform useful actions is also critical in a survival mode. All martial arts classes, existing martial arts or otherwise, must be trained for this. In a real manner, the adrenaline is flowing, and your thinking may fade. If the person threatening you is using drugs or is crazy about most criminals, they are probably not thinking straight, so your ability to consider is a huge advantage.
Fitness
Fitness is a huge plus in keeping you safe, and shouldn't be underestimated. Attackers often have second thoughts before going after someone who appears to be fit and resilient. The confidence you gain through some self-defense training shows in your influence and acts as a deterrent.
The flip side of staying in shape is that you have the stamina and agility to avoid noticeable bumps and knocks. It's a great advantage to miss a shot or to be able to outrun a striker!
Another side effect of fitness is that it seems to affect awareness of your surroundings. You tend to look more and notice things if your attention is not limited to moving a weak and tired body!