Trying to loose weight?

I am 5'9 and weigh about 210lbs, I am told that a healthy weight for me would be around 135lbs. SO I have just bought a gym p and im meeting with a personal trainer and nutritionist tonight. I plan on hitting the gym 3 nights a week for at least an hour a week. How long should I give myself to reach my goal of 135lbs? Is there anyting else that I should be doing to help me achieve my goal? how can I stay motivated so I can reach my goal? Should I go on a diet in conjunction with my exersize plan, if so, what should I be eating? Thanks for all the help.