Can someone help me with WordPress?

So I like this theme I'm using, but my header looks terrible.

I want to have a larger, higher quality image which covers the whole length of the header. I also don't know how to remove that search bar at the end, its not in the widgets.

Any advice, fixes or plugins you can recommend?

(p.s. everything I have learnt about WP has been in the past week, so please be gentle)