How can I convert my design files to Gcode?
What software are you using to design? I use Fusion 360 and there is an option in the menu to send to your preferred (configurable) slicer. Cura in my case. I'm not familiar with other CAD software but I assume others will have a similar function.
Cura is a free option (suggest you use v15.04)
Or, Octoprint has a slicer built in that uses the Cura engine. The actual Cura will give you a bit more control though
Why v15.04? I use Cura 3.3.1 and there is an Octoprint PlugIn for it that allows you to send your STL files direct to OctoPrint as GCODE. They appear in the files list within OctoPrint quite seamlessly. It really speeds up the workflow and the editing facilities in 3.3.1 are vastly superior to 15.04 or the Cura engine in OctoPrint. all IMHO.
Matter of convenience I guess. If you install 15.04 Octoprint will import your settings from that program. 3.3.1 may be superior, but if you're new to this then all the options might be confusing, and maybe even a little overwhelming
I've got 15.04 tweaked for my printers, and this way, if I want, I can slice right from Octoprint, and I know my settings are already in there
The way I have everything set up, I can be on my phone, find a file, slice it, and start the print all while sitting on a bus if I want
To each their own. 
I assumed the OP was using a Mac or PC as he was using design software. I've found getting stuff from my PC to OctoPrint using Cura 3.3.1 absolutely painless.
I have to agree with @Norman, using Cura 15.04 just to be compatible with the slicer bundled with OctoPrint is too restricting. A more recent slicer on your desktop (or cloud-based) will give you substantially more options and features (when you learn to use them). Most modern slicers have defaults that are pretty good so you aren't overwhelmed with options when you are beginning. The ability to separate printing from slicing (i.e. each on a separate system) will prove to be advantageous.
As @Norman points out, more and more slicers are including OctoPrint integration which makes the process painless.
I use Cura 3.3.1 because I love what relative extrusion can do for me.
All right all right, I admit it !
It hasn't been me who's been doing the 3D printing or any of these projects. I've just been typing for the REAL person who's been doing it cuz he can't type...
My cat
And now he says to tell you that you can't teach an old fart new tricks
Now he says to tell you that while he can't use a keyboard, he sure as heck knows how to abuse a mouse
Now he says to say "Get off my lawn"
I used willcom software for my embroidery design work.i want to convert that design to Gcode.willcom generate DST file, that file I want to convert into Gcode.i want that software details.
A Tajima .DST file from the world of embroidery is essentially a 2D graphic. I think I would look for a DST-to-JPG converter and then bring the JPG file into a program for creating 3D work (I use Autodesk Fusion 360). This program creates STL files and those are sliced with something like Cura into a GCODE file.
Google: convert DST file to JPG