Determining your ability scores can be done a number of ways. Some prefer the point buy system from the Player's Handbook. Others prefer to take a pre-made set of ability scores out of the same book. And other people like to roll for their stats.
Follow these rules below for the stat rolling method created (and made popular) by my friends!
Step 1.) For each stat, plus one extra, you will roll four d6's. Adding together the three highest dice and discarding the lowest. This will give you seven, hopefully decent stats.
Step 2.) Discard the lowest number of that set. So if you end up with: 16, 14, 10, 13, 18, 8, and a 12, you will discard the 8 and use the other six. This gives you your first 'set'.
Step 3.) Repeat Steps 1 and 2, twice more.
Step 4.) You should now have three different 'sets' of stats. Pick whichever one you want to use and discard the other two.
Step 5.) You can move points from ONE stat to ONE other stat. You can NOT exceed 18 in one stat. For example; if I took that set I used before then I would take 2 points out of the 13 and add them to the 16 to create an 11 and an 18.
Step 6.) Apply each stat to whichever ability you wish.
Step 7.) Apply racial modifiers.
Get it? Good! Also, ability scores can not exceed 18 (before applying racial bonuses).
Good luck, everyone!