Right click on the grand exchanger that's yellow, not blue, choose sets, right click on the mithril set and choose exchange
These can simply be bought in any Armour shop.
They usually are sold on the Grand Exchange.
Yes.
no, there is no way to make any kind of trimmed armour, it can only be bought.
you're sure it's steel?.. and are you sure your def. lvl is 8?
Most armour and similar clothing is best bought and sold on the Grand Exchange. Note that you'll only sell if another player buys; that means prices can vary, and you may not be able to sell immediately.
Bone weapons can be easily bought from the Grand Exchange at a reasonable price.
Barrows armour can be obtained by trading with another player if fully broken or undamaged as well as being bought on the Grand Exchange. The "Barrows minigame" involves fighting 6 different brothers and defeating them. Their is a small chance of receiving a piece of the brother/brothers armour if you have killed them before looting the chest... For more info search barrows runescape on Google
I'd say that it would be brand new, because you would have bought it for the price of a brand new one, instead of used.
Items bought from the grand exchange will either be noted, or in a box will the full set of the armor you have bought if the item is noted this is simply solved by putting the item into your bank, and withdrawing it again. if the item is in a box, you must first talk to one of the grand exchange people and exchange your box with your items in for the actual items, you then will be able to equip any item.
You can't make them. They can be bought in the Grand Exchange or they are rewards from Medium treasure trails (Clue Scrolls) which can only be obtain by members.
you can buy some from the general store at the docks near Fallador, or if youre a member you can farm them using farming