Are there any big fees for it?
Just the transaction fee which is required for every transaction. Since the Bitcoin network usage is quite low right now, you should only spend a few cents. My last transfer costed me 10c.
i have downloaded exodus wallet , but there are asking 12pharse words. but we only have private key. so what is that differnace?
Every Bitcoin address has a private key. If you have the private key, you can spend the coins of that address.
Some wallets manage multiple addresses (usually a new address is generated for every transaction you receive, for privacy reasons), so they give you what they call a "backup seed", a list of words which represents what takes to generate the same addresses. It's used for backing up your wallet.
Seed -> multiple private keys -> each private key -> address