Gonna assume you're American from the flag you are posting under. You can use a gift credit card or an actual credit card. If you have your own credit card it's likely you are old enough that nobody that matters (to you) is going to peer into your buy records.
Some websites can ship to a PO box, especially domestic companies. Some websites also ship with minimal/discreet packaging to lower suspicion. Bad Dragon, for example, does all of the above, but they don't ship to PO boxes outside of the US.
>How do you know if it's clean/safe
If a site has customer reviews or FAQ on toy quality, good to read. A lot of this boils down to your best judgment and common sense. If you still wind up concerned with what you bought, put a rubber on it.
>size
Again, best discretion. Try not to let your eyes get bigger than your ass.
>how do I do this legally
It is not illegal to buy sex toys in the United States or most civilized nations. Actual no fun countries like Morocco have forbidden sale of these items. Personal research would be required if you really want something of that nature in such a country.
>legal oversights
Canada has some weird laws over what constitutes CP. If you are buying something with or like a depiction of someone underage they may block the item from arriving or impose charges on you. These instances are rare but if you think it could be a problem then it's probably best you don't buy it.
>onaholes
As far as selections, there are entire communities dedicated to these things. Take care of your toys. Guides exist on good toy care. If you don't take proper care of them whatever infection you incur is your own fault.
>websites to avoid
None in particular but I don't care for Doc Johnson toys. They don't have a very comfortable shape.