Okay I am going to keep it simple so as not to overwhelm you. After you have it for a week and understand things better you will most likely want to go in and tweak things to better match your goals.
First, I would just wear/use your Fitbit for about 3-4 days without worrying about changing anything in your pattern- it helps to see what your old "normal" day is like before you start new habits so you can compare the two- both on the dashboard and how you feel. (For example I had no idea I walked so little on an old normal day- that taught me to make steps a priority early and let that change be the base for my new habits- you may have a different priority area). I would try to cover both weekdays and a weekend so you could see the difference.
Second, the one thing to change early (though I would do it on about day 2 or 3 to compare) is to go into settings (cog/wheel looking thing on upper right) and change the Calorie estimation to disabled. (This is also the page to set your time zone- which some people find hard to find). Make sure you enter your height and weight on this page correctly- that is what is used to determine your base rate for calorie burning. It will take several days for the transition from Fitbit guessing what you are burning to what you are actually burning to become more accurate- so do not stress over the numbers your first week.
Third, what I think is most confusing at first is the "calories over" parts. After you set your weight loss goals you will have several numbers around- you have the calories you have burned, plus the estimate of what you will burn the rest of the day. You will have amount of calories you have logged plus an over/under number based on the estimates for the rest of the day. The calories in vs out tile tells you right then based on what you logged for food and what sync says you burnt (even the base calories you burned while sleeping). It gives a simple visual- if you are under you have a room for some more calories, if you are over you should go move more. The over/under is based on the difference you are seeking (-500 or -750 or whatever you have set as your deficit to lose weight.) The calories one is just how much you have burned off. The food plan one with the plate just keeps showing how many calories you have gone over or under. If you click it though you will eventually see the weeks worth of plates at the top- and I have one red one that says "Over 37 calories". I did not eat 37 more calories than I burned-- I ate 37 more than the deficit I set- so it is still negaive 700 calories for that day. I just know that the next day I have to try harder. You can change those individual goals by clicking the cog on the tile. Be careful in the evening though- if you have been active and suddenly you go sit, the food plan will adjust to the lack of activity as it tries to guess how much you you will be able to eat on your food plan for the rest of the day. So you may have "120 calories left" and then you go sit down come back and be right on target- because you are no longer burning at the pace you were. Because it is early here and I have just 1 hundred steps by food plan tile says I can eat just 970 calories today- but by increasing my activity and steps during the day that wil rise.
Best wishes- just play around with it for a week and come back and ask more questions if you need!
