The SRD entry on monsters as races should have everything you need.
The short version is that an Aranea has 3 racial hit dice and +4 level adjustment, so you'd need to be level 7 to start as one*.
Their racial bonuses to stats are: +0 Str, +4 Dex, +4 Con, +4 Int, +2 Wis, +4 Cha.
They get 2 feats from their Magical Beast hit dice, plus Iron Will as a bonus feat. Note that, as you gain levels, you gain ability points and feats based on hit dice, not ECL, so you'd gain an ability point the next time you level (and, thus, have 4 HD, one of which is in a class) and a feat along with your 3rd class level (at 6 HD).
* NOTE: Savage Species is a 3.0 book with guidelines for converting monsters into "classes", so you could start as an "Aranea" at level 1,
and level up with the party. They're ... of mixed success in my experience.