If you are only flying <25K miles a year, you will not get status. Status means being an "elite" frequent flyer, entitling you to certain perks like free checked bags, upgrades, reduced or waived fees, etc. (Even as the lowest level of elite, your benefits would be limited.)

It is difficult to impossible to transfer points between programs, so having miles on a program without having enough for an award is almost useless. You should choose an airline from each alliance - say United from Star Alliance, Delta from Skyteam, and American from Oneworld - and see that all your points are accrued on that airline. (Do pay attention to making sure your miles don't expire; there are many tricks to keep that from happening.)

For most non-frequent fliers, most of the points you will get will be from credit card signup bonuses, or credit card spending. Explore FT to find the best offers available as to bonus, spending requirement (if any) to get the bonus, and annual fee (usually but not always waived the first year).

Delta has a well-earned reputation for being difficult to redeem miles on. While nominally you can get a domestic round trip for 25K miles, in practice it will very often take 40K.

When people speak of *A, what they mean is that if you achieve certain status with one Star Alliance carrier, you will get benefits on others. Usually it requires Gold status (50K miles flown a year) and even then it will mostly get you things like priority boarding, free bag check, and perhaps lounge access, it won't usually get you upgrades on partner airlines.

So just always credit your miles to one of the three carriers noted above, and if redemption is your goal, prefer Star Alliance or Oneworld to Skyteam.