As with everything, it will depend on your situation and the scope of your planning.

I live in a decent sized city and typically don't need to worry about medium term self care, although that might be something to consider if you're in a remote area where help is some time away.

For what I am most likely to encounter, I normally carry items to treat things that either don't require assistance (minor cuts and abrasions), or things that can be life threatening even in the time it takes an ambulance to arrive, such as car accidents, shootings / stabbings, and so on.

For the first consideration, a typical booboo kit you can find at any store will suffice. PPE (gloves), Bandaids, antiseptic/antibiotic, OTC pain reliever, that sort of thing.

For trauma though, I carry an israeli bandage, combat gauze, CAT tourniquet, and HALO seals in a cargo pocket, which can buy time until help arrives. Sometimes I wear jeans and don't carry everything, but in those cases I still at least carry nitrile gloves at a minimum and the other items are in my cars glove compartment nearby.

If my situation changes and I'm going venturing out into the woods (for a dayhike for example) I would carry additional items depending on how long I would be gone. Most of these items are hygiene related -- bug spray, sunscreen, lip balm, gold bond powder, bordeaux butt paste, anti-chafing stick, TP, and wet wipes for a field bath. First aid items would include items for situations I'm most likely to encounter, like an ace wrap for wrapping an ankle, instant ice packs, anti diarrheal, etc.

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Training is important and as others have said you should strive to improve your skill set, but by all means learn all you can even if you don't have the money for something like a first responder course.

One of our EMS guys mentioned that almost 90 people showed up for a free CPR/AED course that was held at the local university. He said the impact to the community was even greater than those who showed up though, because many of those people will go home and tell their friends and families about it and show them what they learned. No the others won't get a card in the mail saying they're CPR/AED certified, but they still gained enough knowledge that they may be able to save a life.

I'm not saying go play doctor because you saw an emergency cric on MASH or grey's anatomy, and there is no substitute to learning from a professional, but a lot of the first aid stuff can be learned on your own even if you can't afford to take a class.