Do I need to pay taxes if I sit on (do not use) bitcoins? I've done some reading on the IRS bitcoin guidelines and I'm a bit confused how the capital gains taxes works. If I buy some bitcoins in Jan, the value of the bitcoins double by the of the year but I still keep them, do I need to pay a tax on those? Or do I only need to pay tax if I buy something with them/convert them to fiat?

I'm in the USA by the way.
No. Assuming you are not a miner or receiving bitcoin in exchange for services, there are no taxes that you need to pay on the bitcoins until you sell or transfer them. When you sell or transfer them, you need to pay taxes on the difference between the value of the bitcoins and your tax basis in the bitcoins.

The value of the bitcoins would be what you sold them for if you sold them. If you traded them or did not get full value for them the value for tax purposes is their fair market value.

Your basis in the bitcoins is what you paid for them. If you mined them, it's probably their value at the time you acquired them assuming you treated them as income. Otherwise, it gets complicated. (In particular, you should either treat your basis as zero or consult a tax professional if you sell or otherwise dispose of bitcoins you received as a gift!)

If you held the bitcoins for more than a year, this is a long term capital gain. If you acquire and sell bitcoins on a regular basis, the rules get tricky for which sells you have to match to which buys.

I am not a lawyer. Please check with your own tax professional if you need advice you can rely on.