Best way of loading player data?

I’m in the middle of making a platformer game, and I’m wondering what the best way of loading/saving levels and player data would be.
Any suggestions? I mainly just need to know what I am theoretically supposed to do. I’ve never ran into having to read and load files, most of what I’ve done have just been ‘play-through once and see how long it takes till you die’ games, like Mario, for example. ;D

All help is appreciated. :wink: