i have a .stp file but i can not open with any software. help me please!

What is your software?
I use only the solidworks.

unfortunately, your file is incomplete or corrupted.

Neudipr is probably right.
The problem with the file is with the header information. Two screenshots are below. One shows "normal" information typically found in a step file. The other shows the header for this step file.
Your file is either corrupted, or encrypted, but I can't think of any reason why someone, or some program would only encrypt the header.

I also had problem opening a .stp file and then I used a software called FreeCAD that helped me view it but I didn't use it for long so I don't know how well it works. You can find it on the following links. https://www.freecadweb.org/wiki/Download or http://www.softpedia.com/get/Multimedia/Graphic/Graphic-Editors/FreeCAD.shtml

Both links work and the file will take time to open, depending on your processor, mine was an i3 processor with a 4GB ram and it took about eight minutes.

Good Luck