My suggestions as a learner. 
Merhaba, is correct for ´hello´.
Nasılsın? is correct for ´How are you?´, when you ask someone you know.
Iyiym sen? should be Umarım iyisindir or inşallah iyisindir (The ´dir´ at the end is optional) for ´I hope you are well´.
Seni Düsüyorum. ??? My guess Seni hakkında düsünüyorum for ´I am thinking about you´. (I´m not confident about my answer here.)
Umarım yakında seninle konuşacağım. I hope I will speak with you soon. There are probably more colloquial ways of saying this, wait for a native speaker.