Un sueño in Spanish means a dream and, that's why many of my students are quite surprised when I tell them that...
Tener sueño means I'm sleepy or I want to sleep.
In Spanish, we use the verb Tener (to have) to express ideas such as:
Tengo hambre - I'm hungry
I hope you find this helpful!
Remember, when you use to be in Spanish you will probably have to choose between ser, estar, tener and haber.