how do i create an index buffer for a large mesh? i mean specifically how to i set up the array of int's from the vertex data? like what is the logic behind this? i understand what the index buffer is for and how to use it but i dont know how to set up the initial array you will pass to the gl element array buffer.

Not sure if you nean simple geometry that you manually type the numbers for or imported geometry from a 3D program but if it's the latter then it'll already be exported with indices.
If the question is about how it works then the index is simply the location in the vertex buffer to use the data from.
Example: vertex buffer
x1,y1,z1 (position)
x2,y2,z2
x3,y3,z3
…
Then in the index buffer a triangle would be defined as [0,1,2] and another as [1,2,3] using vertex 1 and 2 twice thereby saving space and bandwidth.