Create an account with TinkerCAD on their website:
Then complete the 6 tutorials they have (shown below). That'll get you the basics down.
All together this should take about 10min.
Clicking import will allow you to bring in .STL files you already have on your computer either ones from Thingiverse or ones you've created in Solidworks. This is the unique tool of TinkerCAD allowing you to morph together files and create items you wouldn't be able to otherwise.
Clicking export will allow you to save your object(s) as an .STL file. If you want only one file make sure all objects on your work plane are "Grouped" together. Ungrouped objects will be saved as separate files.