Specific -Dont say 'to loose weight' say how much you want to loose, ie 6lbs
Measurable - Set a goal that you can measure your progress, ie, 'i want to loose 6lbs 'write down how much you weigh at the start, then monitor every week on two weeks.
Achievable - Are you going to loose 6lbs by going to the gym once a week for half an hour? No. Dedicate your time for when your going to use the gym, and DONT MISS IT.
Realistic - Dont set a goal that is completly unrealistic 'To loose 5 stone in a month' is not going to happen. If you do it correctly you will loose 1-2lbs per week.
Time -Set yourself a time in which you will achieve your goal. If you just say 'i will loose 6lbs' but dont say how long you have got, it becomes difficult to motivate and push yourself to reach the goal.
A SMART goal is 'To loose 6lbs in 6 weeks.' Simple!