There should be a bunch of "face" lines in the file (lines beginning with the letter "f") that tell you how to combine the vertices into an object. For example,
f 1 2 3
would mean a triangle composed of the first three vertices in the file. You might also see something like
f 1/1 2/2 3/3
which is a triangle that also includes texture coordinates,
f 1//1 2//2 3//3
which includes vertex normal vectors, or
f 1/1/1 2/2/2 3/3/3
which is one that includes both.
Wikipedia has an article that includes an overview of the format: https://en.wikipedia.org/wiki/Wavefront_.obj_file