SAB/Piano Score

1) To select multiple instruments on creating a new score, from the Home screen (that shows the Scores that you have opened in the past), click on "New score". This opens the New Score dialog.

At the left, you can select between the types of instruments. The voice sounds are (obviously) under "Vocals" and the piano (equally obviously) under "Keyboards". For future reference, if the instrument you want is not in the list, try changing the drop-down at the top. "Common" instruments do not include the harmonica for example.

So, you want SAB: Click "Vocals: at far left, "Soprano" in the middle, and then click the right arrow to move the selected instrument to the right panel, which shows the instruments that are to be added to the new score you are creating. Repeat this for the "Alto" and "Bass" vocals. Then add the piano from "Keyboards". Again, for future reference, you can add multiple copies of a given instrument. In orchestral scores, you might well see one staff shows "First Violin" and another shows "Second Violin".

At this point, if the right panel does not show the order that you want them, you can click an instrument in that right panel and use the up and down arrows at the far right to move the selected instrument up or down in the list. Ordinarily, this collection of instruments would be listed, from the top, Soprano, Alto, Bass, Piano. I think that will be the default order, but if not or if you want them in a different order for some reason, just move them. You can move them later if you want as well.

Click the Next button and select the key signature, time signature, tempo, and number of measures you want. You may also type the title, a subtitle, the composer, lyricist, and copyright info if you want it. All of this info may be added or changed later if you wish.

Once you have all the choices made that you want to make to start off, click the Done button and you have a blank score with the instruments you have chosen.

2) Alternatively, you could user the template I described. On the New Score dialog, choose Create from template.

Lots of different choices here. For the one that you want, select "Choral" on the left, then "SATB + Piano" in the middle. Click Next to select key signature, time signature, tempo, number of measures, title, subtitle, composer, lyricist, and copyright info if you want to. (Remember, all of this can be selected later if you don't do it now or changed later if you do.) Click Done and you will have a score that includes all the instruments you want, with the additional, unwanted "Tenor" vocal instrument.

To remove it, go to the Instruments tab at upper left. On this tab, however you created the score, you'll see the instruments you have selected, listed in the order you selected. You have several options to remove the "Tenor".
a) You could simply remove it completely (which is what I've assumed you want). Click on "Tenor" to select it, then click on the "trash can" icon near the top right of the tab.

b) If you think there is some possibility you might add a Tenor in the future, you could simply hide the Tenor staff, then unhide it later when it comes time to add that voice. To do this, click on "Tenor" to select it, then click on the "eye" symbol to its left and "close the eye". This hides the Tenor staff but does not actually remove it from the score.
c) There is a third option that I'm fairly certain you don't need or want but again, for future reference, you can also hide the staff but still have it play when you play the score. Perhaps your bass gets sick, you can hide the bass but still have it play by clicking the little triangle immediately left of "Tenor". This displays the sub-items, in particular the "Bass clef". It also has an "eye". Close this eye and it hides the bass clef, but still plays the music notated there.

I recommend you play with this a little. Try different methods of creating scores and changing their instrumentation to get used to it. 15 minutes of play and you'll be quite as knowledgeable as anyone on this topic. In addition, the Handbook has a whole section on this: https://musescore.org/en/handbook/4/setting-your-score