Each wallet let you choose fees when making transaction, but usually wallet do it automatically manage the fees in transaction that could be confirmed quickly.
Also, don't forget that transaction fee also depends on transaction size (usually about 0.2KB).
What affects transaction size? Number of receiver/sender addresses or number of transactions in the sender address?
How to spend unconfirmed bitcoins?
You can't spend unconfirmed transactions. You have to wait until a certain number of confirmations until you can send them from that point as well.
Transaction size is effected by the input and output scripts needed (in bytes) mainly. If you click on any bitcoin transaction on a block explorer like blockchain.info and click "show scripts & coinbase" then you'll see the input and output scripts required for your transaction to send.
Actually, you can spend your unconfirmed transaction. But, it's very risky that makes some wallet don't allow this. However, you can spend your bitcoin after that transaction have at least 1 confirmation (but some people says it's better wait until it has 2-6 confirmation)
