What i do if i want a girls number online is i say something along the lines of:
"if im not busy you can come help me shopping, i mean ur just about cool enough to be be seen with me

"
if she says yes then say "well then give me your number silly

!" If shes into you she will hapilly give over her number, which can be used to arrange to meet.
Regarding your overall conversation i would be more playful, if she does something stupid say something like "hehe thats sooo cute

" always remember to use smilie's at the end to convey your playful attitude.
I would also say things like "sooo what are you getting ME for xmas!

". If she responds playfully then you know that shes interested if she replies very negatively for example she says "nothing", then i would just move on she ain't worth your time.