Question: I have a source code folder of a software and it is written in VB 6.0. 

The source code folder contains different files. Which file should I open and compile?

May you please tell me step by step procedure of opening a file with help of VB 6.0.

Answer: 
When you open VB6 the **New Project** window opens by default. Click the **Existing** tab and browse to your source code folder. The **Files of type** file filter will already be set to the default VB6 project file types.

\*.mak - Older VB projects. I'm not sure when this was changed, but VB3 still used this extension.  

\*.vbp - The default VB6 project file extension  

\*.vbg - The default extension for a group of projects

If you have a \*.vbg file, you should have multiple \*.vbp files and the \*.vbg file is probably the file you want. Otherwise, unless you are opening a really old project you will probably open the \*.vbp file. If you have multiple \*.vbp files in your source code folder without a \*.vbg file present open the \*.vbp file with the name closest to the exe, or dll file you are trying to compile the code to, if you know it.
Open the project file (.VBP) in the VB6 IDE. `File` menu, `Open Project`

Suggestion: read some VB6 tutorials...