Please help with a few basic questions

I can't quite figure out how some of the mishaps happened here, but lets just start from the beginning:

1. The first measure is an upbeat containing only two crotchets (quarter notes). So you need to change measure properties: right click (or control-click in Mac) on the measure, select measure properties and set it to 2 quartet notes. Also set the measure number to zero. I am afraid you'll need to re-enter the notes; begin with the rest (you have a rest after the three eighths in your "original").

2. You can not delete rests. If you delete notes they don't disappear entirely. Rests are substituted for them. The measure will always have enough rests in it to account for its full value.
Hence in the first measure (the upbeat measure is zero) in the right hand select the first sixteenth rest. Type or click N. Enter a c quarter note. You'll see that both rests and your eighth note will disappear. Select the now note and type shift E.

3. In the left hand you have two quarter rests which somehow moved out of place. What you need is a half rest. Select the first quarter rest. Type 6 or click the half note symbol above. Now you have a half rest and measure 1 is ok.

4. Your second measure has an incorrect rest in it. Also it opens as a 5/4 measure in my system. Check the measure properties (see above) and set the measure back to 4 quarters if necessary. Then delete the second half of the right hand and reenter as above or as per handbook.

5. For the left hand of measure 2 select the rest. Type 6 or click the half note symbol. Now you can enter the two notes.

I am under the impression you could profit from a good study of the handbook. So this is my first recommendation: Read the handbook, not everything at once, just the section on note entry for starters, then the other sections by and by. Keep it open while working on your score and go slowly. Musescore is not hard to use and you will get better soon.

There is a second recommendation: From some of your errors which you appear to have overlooked (like the unnecessary rests in measure 2 or at the end of the upbeat measure) I also get the impression that you are not (yet) a very fluent reader of music. Which means you have to be super observant until you get more familiar with it. If you don't play an instrument (or sing from sheet music rather than from memory) I am afraid this is not easy. Maybe you want to take lessons in piano or something for a while or you might join a choir. That would speed up your familiarity with notation. Then again I might be totally wrong about this.

Double post, see https://musescore.org/en/node/100091