The vast majority of people can be hynotised. You will feel the benefits of hypnosis whether or not you think you can be hypnotised.
During hypnosis, you may feel deeply relaxed and focused. Some people experience a sense of weightlessness or heaviness, while others feel a gentle warmth or tingling sensations. Your experience will be unique to you, and you will remain in control and aware throughout the process.
You won't be asleep. You may find that your thoughts drift but your subconscious mind is alert and will be listening to everything that is said. 
Yes you will be in control. You cannot be hypnotised against your will.
It is difficult to predict how many sessions will be needed although I aim to get results as soon as possible.  Most people find they have made significant changes after 5 sessions and some as llittle as three.