HP and multiclassing

[YrdBrd]

Total noob question, but I cannot find an answer. When you multiclass, do you gain the full HD + Con the first time you multiclass into another class or do you only get what you would normally get beyond 1st level? For example, Im thinking of multiclassing my level 1 fighter into monk at level 2. With a Con of 13 and gaining average HP (i.e. half the value of the classes HD rounded up or down depending on odd or even level) where would my HP be? Ive figured either 21 if you get full HD or 17 if not.

[Halfling Barbarian]

You roll for hp every character level after 1st, even if it's a first class level. Since you're going with average hp it would be 17.

[Brox RedGloves]

You'd gain the full HP for a level one fighter (11) and a d8 hp for your second level as monk. So you'd be at 17.

[ShadowcatX]

You get the average, so 17.

[YrdBrd]

Thank yall very much.