When App shows several different number of calories for a food

If the bread label is available, use that. You can create your own Food using the label, or you can use one that is close.

If you're eating a prepared sandwich like at a restaurant? If the restaurant doesn't post its nutrition and calories then you'll have to guess. There are legitimately many different calories for rye bread depending on the portion size.

One of the benefits of preparing my own food and logging is that I can now guesstimate when I eat restaurant food.

How often do you eat out, and do so in restaurants that don't post calorie information (non-chains, in the US)?

If it's not very often (couple of times a week, say, on average), make your best estimate, log it, then go on with life. It will be close enough. As Riverside mentions, you'll get better at estimating portion sizes as you gain experience logging home meals, ideally using a food scale there at home. (Some of us game-ify scale use, guessing the weight before we look at the display. ).

In non-chain restaurants, I usually do one of 3 things:
1. Find a similar food from a chain restaurant, and log that.
2. Deconstruct the dish (which it sounds like you may be doing, estimating bread separately from sandwich fillings). When I do this, I usually choose middling-high entries from the database, not the silly lowball ones, but not usually the highest of the high, either.
3. Use other people's estimates in the database for similar meals, preferably choosing ones that have an intelligible serving size (inches/cm, cups/ml, something like that - not "1 piece" kind of thing, ideally). Here, I'm talking about things like lasagna, or other things that are hard to mentally deconstruct. Again, I'd choose a middling to high one, if there were multiples.

If a dish was particularly rich, I might log some extra oil/butter, since restaurants sometimes slather that on.

If it's in a context where it's polite, I might snap a discreet photo of my plate with my phone (no flash!). I usually put a fork or something of standard size in the photo, so I can use its size to estimate portions later.

I admit, when I eat out, I usually choose the food I truly want to eat . . . but in a case where there are multiple good-looking things, I might choose the one that's easier to estimate calories, all other things being equal.

Since you're new to this, I can understand being a bit anxious about some of these cases. You'll learn more, be able to estimate better as you go along, and low estimates will tend to balance out high ones, as long as we don't intentionally lowball, I think. MFP may reset at midnight, but our bodies don't, so the averages are OK to think about.

Also, if you told MFP you want to lose a pound a week, there's 500 calories between your daily calorie goal and your "maintain current weight" calories. (Half a pound a week, 250 calories daily; 2 pounds a week, 1000 calories daily.)

Anytime we eat below the weight-maintenance calories, even if above goal, we can expect to lose weight, maybe just a tiny bit slower than anticipated. For example, eating 200 calories more than goal on one day is only going to make about 6 hundredths of a pound (roughly 0.9 ounces) estimated slower weight loss that week. It's not worth stressing over.

Just make a reasonable estimate, and go on. It'll be fine.