Beginners should start with an easy selection.
What's easy?
A Red Frisbee™ on green grass?
Yes!
A Multi-colored snake slithering across multi-colored rocks?
No!
You often need to use more than one tool to make a selection.
For example, the Selection Brush tool is often used to "mop" away unneeded parts of a selection.
Go to Selection Brush Mop.
You can alternate between the Lasso tool (operates free hand) and the Polygonal Lasso tool (makes straight lines) by pressing and holding Alt.
Enlarge the area that you're selecting with the Zoom tool.
And, realize that minor errors probably won't show when the photograph is at 100% magnification.