Well, they don't actually sound the same, although you may need to practice to "tune" your ear. The consonants are the same, but both vowel sounds are different in the two words.
For one thing, it isn't "débout", but "debout" - the "dé" in "début" sounds somewhat like the English word "day", while the "de" in "debout" is much shorter, rather like the "de" in "defer".
The second sound in "début" doesn't have any counterpart in English; it's not a sound we make. You can try, though, by pursing your lips as if to say "ooo" and trying to say "eee" instead. The second sound in "debout" is sort of like "boo", although a bit shorter and more clipped.
If you go to http://www.acapela-group.com/, you can choose a French voice, enter "début; debout" into the box and click on "listen!" You should be able to hear the difference. Try more than one voice.
Hope that helps.
The correct grammar would be « Je préfère être debout » (as a general statement of your preference for standing compared to sitting) or « Je préfère rester debout » ("remain", when you are already standing and someone was offering you to sit).
If you were asked : Would you prefer standing or sitting? => Préférez-vous être debout ou assis?
you could answer : Standing => Debout. But you wouldn't use the exclamation point in that case.
This is not actually an imperative. It may be a shortened version of one, however. Here is a link that might help:
It's a bit like saying just "Up!" to someone instead of "Stand up/get up!"
But to your question, imperatives comes in three forms: vous/tu/nous. Depending on whether you are addressing someone with a formal or informal relationship, whether it is a single person or multiple, and whether you are included in the command. So for instance, for "aller" the forms are allez, va and allons (the s is dropped from the "tu" form of -er verbs).
There are also a few irregular imperatives such as soyez/sois/soyons (for etre).
Aha, good call, that lead me to this reference you might be interested in; https://books.google.com/books?id=9Uz2VHiGoNMC&pg=PA141&lpg=PA141&dq=Mets-toi+debout!&source=bl&ots=DfoBE2tII0&sig=_32qfe21v7ujXkH6LY7qCe7oj94&hl=en&sa=X&ved=2ahUKEwiE8NCF9KXfAhUqw1kKHXnoDygQ6AEwFHoECCgQAQ#v=onepage&q=Mets-toi%20debout!&f=false.