How to include header files while compiling a program in ubuntu ? like string.h etc.
I know for compiling with include<math.h> we need to use gcc filename.c -o programname -lm
but what about others ?

You can easily run them using C++.
In ubuntu, first install the g++ compiler using terminal:

sudo apt-get install g++

now go to the folder containing your C++ file and compile it using:
g++ fileName.cpp

and you can run it by typing ./a.out

why go for C++ compiler? Doesn`t C have these header files? Sorry.I am new to programming.

You can use it in the same way you included the math.h header file.

#include < string.h > 
#include < math.h >
#include < stdio.h >

gcc filename -lm.

this compiles any function of math.h and i think this solves your issue

for string.h and others simply include the header files it will not cause any trouble. only it is aused by math.h header file.