1) Whatever you do in the past (or present when you get back from the future) will effect the future (creating an alternate timeline/universe)
I'd go back to when I bought my new car and tell myself that the green in the brochure won't look as good on the car itself. Get red, black or white.

OR

Knock that first cigarette out of my hands.

2) Whatever you do will not in anyway effect the future (so its an observatory trip to see what mightve happened if you changed something in the past or what will definitely happen no matter what you do if you go to the future)
I'd probably make a lot of big plans to experience the world, do all sorts of things I'd missed out on etc. but end up playing video games 24/7.