Instructions
Learn spells during character creation and while leveling up. "The Player's Handbook 2" features a list of sorcerer spells ranging from feeble Level 1 varieties to the most powerful spells available at Level 29. The character cannot know all of them and must choose the ones he wants from the list.
Select the spell you want to cast during your turn. You can also cast spells out of combat at your leisure.
Check to make sure that you're ready to cast the spell. Most spells have a limit use, such as once daily or once per encounter. If you've already used the spell within that limit, you cannot cast it again. Select another spell, instead.
Announce to the game master that you intend to cast the selected spell. So long as he doesn't object, roll the dice associated with the spell. Typically this involves rolling the number of dice you get for your charisma bonus, as sorcerer's draw upon their charisma rather than their intelligence. Refer to the spell within the game manual to verify.
Note that you've cast the spell if it has a daily or encounter limit. The simplest way to do this is to put a check mark next to it on your character sheet, though if you'd prefer to keep the sheet pristine, jot a note on a separate piece of paper.