You don't roll abilities with a 20-sided dice. Not in Dungeons & Dragons.
If you look at Section 3 - Determine Ability Scores (p.12 of the Player's Handbook or p.7 of the Basic Rules) it lists three methods of generating ability scores. The simplest is what we call the "Array Method", which doesn't involve rolling dice at all - you take the six numbers 15, 14, 13, 12, 10, 8 and allocate them to your six abilities in any order you like. The description of the class you have chosen will suggest which abilities are the most important; you put the highest numbers (15, 14 and so on) into those.
If you really want to roll dice, you need six-sided dice to roll ability scores. But check with the Dungeon Master of the game you are going to play in - some Dungeon Masters do not allow randomly rolled ability scores.
What class and race have you chosen for your character ?