I don't know how to sarte.

Before I even started I wrote down everything I ate in a day, after a couple of days I was amazed at the amount for food and calories I had consumed, it made me accountable for what I was eating. I slowly started to make better food choices and kept on keeping track of what I was eating. I also started walking 10 minutes a day then increased it by 10 minutes until I was up to 30 minutes. My weight loss is still a work in progress but I am well on the way to a fitter and healthier me. Good luck!

Start by filling in your goals. (My Home tab > Goals)
You can tailor them if you have specific targets, but otherwise I'd start with just the default MFP recommendations.
For some people starting with many goals and a whole lifestyle makeover works, but for many people (and I'm one of them), it's easier to work on just one or two things at a time. One path would be to take the following steps, progressing as you are ready. You may already have some of the following under control.

1) Get your initial statistics -- weight, measurements.
2) Enter your goals. (Under the My Home tab - Goals)
3) Record any initial targets, goals or motivations. You can put these in your MyFitnessPal profile or write them somewhere else, but it is a good idea to write them.
4) Start recording what you eat - everything.
5) Add more healthy foods to what you eat.
6) Start recording your exercise.
7) Plan some rewards for reaching your early, interim goals.
8) Stop eating some of the less healthy things you eat.
9) Increase the amount you move each day.
10) Work on portion control.
11) Walk more.
12) Try a different kind of exercise (in addition or to rotate, consider stretching, aerobics and weight bearing exercises).
13) Record your progress.
14) Ask for help on any parts you're finding difficult.
15) Adjust your interim targets, goals and rewards.
16) Drink at least 8 cups of non-alcoholic liquids each day.
17) Try some other healthy foods.
18) Increase your exercise.
19) Record your successes.
20) Help someone else get started with all the things you've learned as you've become more fit.