I am building my home, but now I need 30 quarried stones. Where can I find these?

you can mine them on your property

And there is a pickaxe right next to it.

Thanks. How do I buy furniture?

You can make your own furniture with wood and nails and other stuff at the workbenches inside the house. Or you can ask your steward to buy them for you. I recommend building your own. It's more fun that way.

You are better off going into the Hearthfire forum, all the info you need is there.

To furnish the house you can either:

• Use the workbenches inside the home to make the various items; each item will tell you what's needed to make it, and some materials can be scarce if you're on a PC and have certain mods running
  
• Tell your steward (if you have one) to do it; this avoids any issues with finding materials, but is a lot slower in most cases

Neither is particularly better than the other, although the character's RP may dictate which way you go.

One thing with mining the stone: unlike mining ore, the action of mining the stone (or clay, for that matter) does not stop until you tell it to (hitting 'S' (or the proper controller button for moving backwards) will do so, for one), so if you let it run for a bit you'll end up with quite a bit of stone on hand. As for how to know when to stop: every so often a message will pop up in the upper left corner of the screen saying '4 Stone added' (without the ' marks), and after five of these (which will give you 36 Quarried Stone) you'll have enough.

In case you're wondering: the message is actually incorrect after the first time, because the amount you get per iteration is based on the number of pickaxe strikes. The first one, which is three strikes, gives 4 stone, while the rest are 6 and give 8. As such, 5 iterations are needed, because 4 will only yield 28 (4+(3*8)), which is 2 short. If you interrupt an iteration you get 4 stones, regardless of how many strikes in you are.

Also, the Steward can purchase Wood, stone and Clay for you thru dialogue.

True, which does help with making a lot of the items, however ingots (or ore, if you prefer to smelt your own) and critter parts you have to go get yourself (mine/hunt or buy, as is appropriate).