What advice would a personal trainer most likely give you before a workout

Well, they would tell you to stretch before you work out, stay confident, and stay focused. Why? Because you need to stretch before a work out to make sure your body is prepped up for it and so you will not have any cramps or injuries during the workout; in order for you to reach the progress you expect to reach, you MUST stay confident or else you'll lose your self esteem and your mind will be like, "I can't do it", and remember that your MIND CONTROLS what your body does, so your body would not succeed in workouts; you need to stay focused because if you misunderstand some instructions or procedures or steps, you could go off in a different direction because you missed some important information. Think of it like a recipe; if you mess up on a specific step, your cake could become from the most DELICIOUS to the most DISGUSTING treat ANYBODY could've EVER tasted! That's why they tell you these things to be prepared. ;-)
I hope I helped! =D

To do your best,but don't put too much pressure on yourself,just only do what you can easily handle and manage

Well. erm obesity, anorexia?