The sorcerer gets a new spell level one level later than the wizard. The wizard gets a new spell level at level 3, 5, 7, 9, and so on. So a sorcerer gets a new spell level at level 4, 6, 8, 10, and so on.
The sorcerer starts with 3 spell slots per spell level per day compared to the (non-specialist) wizard's 1 spell slot per spell level per day. The spell slots generally increase by 1 every time you level up, up to a maximum of 6 slots for sorcerer and 5 slots for (non-specialist) wizard.
Its as printed in the ToB manual (the table is absent from the SoA manual for whatever reason). You can download a manual from sorcerers.net if you don't have a hard copy or one on the disc. You can also find them here: http://playithardcore.com/pihwiki/index.php?title=Baldur%27s_Gate:_Progression_Charts