How do I get the name or number of device?
Determining the exact part numbers is part of doing electrical design.  After a while, you will get to know some parts, particularly general purpose ones.  For example when I need a "jellybean" transistor, I generally use a MMBT4401 for NPN and MMBT4403 for PNP.  These are widely available, cheap, with reasonable specs, and I know enough about them to know when to pick a part more carefully.

When you do have to search for a new part, doing a parametric search on a good website is the best way.  I like Mouser for this.  They have the best parameteric search in the business, at least last time I checked.  Their prices also seem to be generally better than other distributors, so if Mouser has a suitable part I'm usually done.  Next I go to DigiKey because their web site is pretty good too (used to be the best until Mouser fixed theirs), and they have a wider selection.

There will always be parts you don't know and have to look up.  Over time you'll get used to some, but you can never hope to know more than a tiny fraction of what is out there.  This is normal and you shouldn't feel bad about that.