On of the worst things about learning Spanish is that there are two verbs for to be. This is a big unesesary complication but you will never speak Spanish well unless you can use "ser" and "estar" correctly.
For example you use "ser" for professions.
Soy profesor - I am a teacher
Estar is often used for states:
Estoy cansada - I am tired.
There is a good guide here http://lingolex.com/ser/
You can practise the present of SER here
You can practice the present of ESTAR here