Unless you're wanting to have a home-maker (which many power-gamers around here seems to have) you probably want a job for each sim, if anything with similar requirements at least as you won't have to buy training machines for all skills. Then stick with that job; even when you've reached the highest level (unless it contradicts your lifetime wish or something
, your sim can get raises and bonuses until the day he/she dies.
If you want to know the requirements, please refer to
this page and the related subtopics. Some careers (the ones from expansion packs) aren't available to the console.