How to present a numbered list in a sentence

There are several ways to give an enumerated (numbered) list in sentence form. One way is to put the number (not the number word) in parentheses, like this:

I came up with three reasons: (1) this is my first reason, (2) this is my second reason, and (3) this is my third reason.

Notice that the numbers are inside a pair of parentheses. It is not considered correct to use just one parenthesis after the number. If the introduction of the reasons is not a complete sentence, do not use a colon to introduce the list. Instead, do this:

My reasons are (1) this is my first reason, (2) this is my second reason, and (3) this is my third reason.

If the text of any of the reasons contains a comma, then you use semicolons between the items—but if that happens, you should consider making it a vertical list, like this:

I came up with three reasons:
1. This is my first reason, which contains a comma.
2. This is my second reason.
3. This is my third reason.

Another way to list reasons would be in separate sentences, using the ordinal forms of the number words, like this:

I came up with three reasons. First, this is my first reason. Second, this is my second reason. Third, this is my third reason.

Combining these into one big sentence with semicolons is not recommended, because if you are putting an enumerated list into one sentence, you use the numbers in parentheses as in my first example.

You can, of course, choose to use letters instead of numbers in all but the last situation.