Will retainers keep my teeth from moving
Retainers are designed to keep your teeth from moving...doesn't matter if the original problem was corrected by braces or you were just blessed with good teeth to begin with
Unfortunately, just because your teeth are straight when you are young doesn't mean they can't move as you get older.
So, yes, retainers will keep things were they are...but since you will need to wear them forever at night, ask yourself if you are happy with the current alignment...if you need to wear the retainers anyway, maybe you want to get your teeth ideal before you make retainers.
Retainers will prevent teeth from moving
Yes, if made correctly and worn as instructed by your orthodontist, retainers will prevent your teeth from further shifting. Remember, retainers are for "retaining", and not necessarily for "moving" teeth. They should do a good job keeping them in their current position, but if you want to move them back to their original position, you should investigate treatment using braces or Invisalign with your orthodontist.