Can I do my first 90 day report online or do I need to go to the immigration office?

if you are going to mail it in to C/W you need to have it post marked 14 days before your due date;

make copies of;

data page of your passport

your visa/extension

your most recent entry stamp

your TM6 departure card

your TM30 receipt

fill out a TM47 90 day report form

include a self addressed stamped envelope so they can mail your receipt back

It's best to send it in EMS so you can track that they received it but you can use just a stamped envelope to get the receipt back

You mail it to this address

ATTENTION 90 Day Reporting

Immigration Office Division 1

Chalermprakiat Government Complex

120 Moo 3 Chaengwattana Road Soi 7

Thungsonghong, Laksi Bangkok

the first time you do a 90 day report you cannot use the online system, you either go in person, send someone for you OR mail it in.