$L_1 = \{a^n b^m a\mid n\geqslant1,m\geqslant0\} $
I can't really seem to figure the list of productions for this simple language, I know I have to write the non-terminals, but how do I decide which is the starting point S? And then how do I continue?
!!Answer to the original question for $L= \{a^n b^n a | n\geq1, m\geq0\}$!!
The starting point is some non-terminal symbol you define as such, let us call it S.
Now we observe that the numbers of a and b in $a^n b^n$ must be equal. The obvious way to achieve this is to generate them in pairs. Because we do not want them mixed, the non-terminal must be inbetween them: $A \rightarrow aAb$.
To generate the final $a$ we use $S\rightarrow Aa$.
For terminating $A \rightarrow ab$.