S-> aSb| A | B
A-> aA | a
B-> bB | b
Question was written incorrect so i assumed question as {anbm|n,m$\geq$0 & n$\neq m$}
S-> $\varepsilon$ is not possible here.
what I did is break language in 3 parts
1) S-> aSb to give equal no of a's followed by equal number of b's
2) S->A so that string contain at least single a followed by 0 b's
3) S-> B so string contain at last single b followed by 0 a's
if string start with step 1 then it must follow either 2 or 3 to terminate
if it follow 2 then language will have a>b else if 3 is followed language will have a<b