When you get input you can change it to higher case so you only check for highercase strings.
a2$=upper$(a2$)
IF a2$="A" THEN gosub scene2
IF a2$="B" THEN gosub scene2a
IF a2$="C" THEN gosub scene2b
If you don't really want to see the text output you can use inkey$(). It has to be in a loop though... inkey$() checks for keys when it sees that line and instantly goes to the next line. You don't have to use different strings for each input either (no a1$, a2$, a3$... all can be a$ or whatever you want).
do
a$=inkey$()
a$=upper$(a$)
if a$>""
if a$="A" THEN gosub scene2
if a$="B" THEN gosub scene2a
if a$="C" THEN gosub scene2b
endif
loop
One of the most important things you need to learn is spacing your code. See how the above snip has the "do" to the far left and the next command under that is one space over " a$=inkey$()". When you nest properly everything should line up. Sven B uses tabs to space code which is just as good. It just depends on your preference. But spaced out code is much, much easier to read... especially when you go back to it months or even years later.
And I hate to nitpick too but "cannibal monkeys" eat other monkeys... not humans.