It really depends on what you want the end product to be like. My basement has bare concrete walls and floor, ~25 60-watt LED equivalent bulbs, a pool table, a wood stove, a couple gliding rocking chairs, and my supply of homebrew. In addition it has all of the usual basement stuff, laundry, boiler, hot water, upright freezer, wire rack shelves, electrical panel, duct work, and so on.
The single biggest for us was to upgrade the lighting circuit form a measly 7 bulbs to our current 25; it happened over time, a light for the laundry sink, a light for the freezer, a light for the caned goods shelf, and before we knew it hey its pretty well lit down there.
Once you have dealt with the lighting, as other have suggested, painting the concrete will also help it feel less cave-like.
Oh and organization; I have been in basements larger than mine and they are not nearly as comfortable, because things are just strewn about or it is just a mess. your can put a lot of stuff on a couple wire rack shelves and it look so much better.
I would call most of the finishing steps of a basement DIY'able (framing, drywall, flooring, and so on). So your budget and how much time you want to spend are going to be your limiting factors.