You are already in a position to learn the HTML key - tags.
"Tags"?

Tags are labels used to tell the browser how to make the website.

All tags have the same format: they begin with a less than sign"<" and end with a greater than sign ">".

Generally speaking, there are two kinds of tags - opening tags: and closing tags: </ command>. The difference between them is that in closing tag there is a slash "/".

All that is contained between an opening tag and a closing tag will be processed according to the command in the tag.

But as every rule has its exception, here in HTML the exception is that for some tags to opening and closing occurs on the same tag. Such tags contains commands that do not require content to be processed, i.e., tags are commands isolated, for example, a line jump is achieved with the tag <br />.

HTML tag is - nothing more than tags. Learning HTML is learning how to use different tags.
You can show some examples?

OK, the <b> tag tells the browser that all the text between the <b> and </ b>is shown in bold. (The command "b" is an abbreviation for "bold."- Bold)

Example 1:

<b> This text should be in bold. </ b>

It will be displayed in the browser, as shown below:

This text should be in bold.

The tags <h1>, <h2>, <h3> <h4> <h5> and <h6> tell the browser that this is a header (h stands for "heading" - header) and <h1>the first level header and one presented with the larger text size,<h2> the second-level header and one presented with a slightly smaller text size and <h6> the sixth level header and one presented with the smallest size text.

Example 2:

<h1> This is a heading </ h1>
<h2> This is a sub title </ h2>

It will be displayed in the browser, as shown below:

This is a heading

This is a sub title

Tags must be written in uppercase or lowercase?

For most browsers does not care if you use uppercase, lowercase or a mixture of them. , or usually has the same effect. But the correct way is to use lower case. So make a habit of writing your tags in lowercase.
Where do I put all these tags?

You should write your tags in an HTML document. A website is constituted by one or more HTML documents. When you surf the Web, you are opening different HTML documents.