One thing you will have to get used to when shopping online is measuring things.
You will need:
A measuring tape
A set of scales
Use every method possible to get a sense of what the item will look like in your house - lounge, bedroom or whatever. How much space will it take up? There's nothing worse than getting excited about a purchase, getting it home and realising it's too big or clashes with your decoration scheme.
When it comes to clothing - make sure the size is relevant to your country - a US size 10 is hugely different to UK or Australian sizes. Another mistake people make with sizes is ordering a size 10 - and finding out the clothes are in children's sizes.