how i get the spells in arena?

The screen just after that one should show you the spells to choose from. Tap on the spell or spells you want to use.

Ayyoya, the opponent you see may be someone who played that set of rooms days ago, and they selected to bring in one spell – a good choice, if you have enough because there’s often that one pig that can upset your whole arena round :)! It’s basically a pre-recorded opponent. It also means they start with 8 rather than 7 cards, which is obviously an advantage.

You can think of your arena opponent as a recording of some other player in the same set of rooms like the ones you are about to play. So he/she played the rooms some other day, and you are about to Play the same rooms now. In case he/she Chose a spell, you will have to battle the birds and the spells. Yesterday I had a battle against a player using three spells (duckies, chillie and eagle) and a higher flock power than mine.
The only way to get this advantage is to choose your spells after clicking on the battle! button.