Either use the 15-14-13-12-10-8 block or use the point buy system on page 13 of the PHB. Note that with the point buy system, the maximum for one stat (before racial modifier) is 15 and the minimum is 8.
Note that we are using a variant saving throw system. All abilities are important. Charisma is not a dump stat.
Maximum HP at 1st level. Each level after that you can choose to take the average value or to roll. If you roll low then you have to keep the value.