External CSS - How do I write/apply tags in HTML page (beginner)

External CSS - How do I write/apply tags in HTML page (beginner)

My External CSS and template was created by a friend fluent in CSS. But my
friend isn't understanding that I don't know HOW to 'apply' (write) the
tags in the HTML page to 'call' the CSS.
I know the difference between id (#whatever) and class (.whatever) But I
don't know if a CSS entry like
.header caption (with styles listed) translates then to 'p class="header
caption" with opening/closing tags ???
What I'm looking for, but not finding on the web, is a listing of how the
most common tags are written as css. For example, I know
p class="left" applies the CSS to align left, instead of HTML tag
align="left" But How do I write the actual tag for .table .td .tr
Is there a website to show: "This is the tag in HTML, no External CSS"
"This is the way to write the same tag for External CSS"
Second question... Other than for the 'p class' or easy tags, do I start
every tag with: class="" ? p class="" ? div class="" ?
When do I use SPAN vs DIV ?
I have searched for how to apply External CSS, for what tags to use--yes,
I understand that every person might use different naming for divs and
classes-- but I really need examples that will show the basic tags so I
can plug in the 'different naming' from my
'external css'
Is there one website that shows (all on one page) examples of the tags? A
website tutorial that doesn't assume novices already know how to write the
tags to apply css?
Thanks Lifes