Characters have a proficiency bonus determined by level, as detailed in chapter 1.
Your proficiency bonus can’t be added to a single die roll or other number more than once. For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once when you make the save.
Depending on your relevant ability score, you can often add 2x your proficiency bonus to an ability check (see the rules for Ability Checks). The base 5e rules occasionally allow you to multiply your proficiency bonus (for example, the rogue’s Expertise feature). These instances will be reviewed under this rules modification. In the meantime, assume any such abilities grant a +2 bonus to the check instead (if more than one multiplying ability is involved, apply this bonus only once: it does not stack).
In general, you don’t multiply your proficiency bonus for attack rolls or saving throws. If a feature or effect allows you to do so, these same rules apply.