What are the important things that need to be done at the initial stages of application testing?

1.
Determine and understand the actual scope of work 2.
Ascertain the equipment, communication protocols and configuration softwares involved.
3.
Gather all relevant documents required before commencing work ie, client approved signal list, Sld, bld etc 4.
Prepare a work schedule after keeping in mind risks involved and regional client safety norms.

I think if you are taking in a generic way, then you should firstly learn what is application testing means.
there's a nice course for beginners called ISTQB, you must take this course if you are new in this field.

According to project requirement the the system testing is done.
First thing is the scope of the project.