Isomers have same formula but the different arrangement of atoms/ structure.
Butane is an alkane which has 4 carbon atoms. The formula is C4H10. 2-methyl propane is also an alkane which has 4 carbon atoms. The formula is also C4H10. Only difference in these compounds is 2-methyl propane has 3 carbon atoms in main carbon chain and there is CH3- group in 2nd C atom while butane has 4 carbon atoms in main carbon chain.
The structures are as the image.