Prim's algm

[SoftDisk]

Hello!
I need some general idea on how to code Prim's algm for a complete undirected graph.
The format in the file is as follows.

[ vertex 1] [ x_coordinate 1] [ y_coordinate 1] [cityname 1]
[ vertex 2] [ x_coordinate 2] [ y_coordinate 2] [cityname 2]
. . .
[ vertex n] [ x_coordinate n] [ y_coordinate n] [cityname n]

So far I loaded the data from the file into a vector of structs.
Then I created a new vector srtucts and calculated the weight of all edges, stored the info (weight from vertex and to vertex) in it....

Now I am not sure if I started right and what to do for the next step...

Any help would be great for me! Thank you!