Write this in onclick event of the button:

var result = confirm("Want to delete?");
if (result) {
    //Logic to delete the item
}

Part of the reason that people skip over long messages is due to reading speed.

Assume for the sake of discussion someone with an average reading speed - around 200 words per minute.(*)

If you use just 20 words in a dialog, you're asking that user to spend 6 seconds reading and understanding what you wrote.

While that doesn't sound like much, a six second imposed pause when you're trying to get something done can seem to be an awfully long time.

(*) And don't make the mistake of assuming that low reading speed means low intelligence.

So, three suggestions for you, all aimed at maximum clarity with minimum fuss.

1. Be as concise as possible
2. Identify the item at risk
3. Name your buttons for the actions

Here is a simple deletion dialog:

Let's reduce the number of words to the minimum to make it easier to read:

Now, let's identify the item at risk, and label the buttons for the action:

Much better - easier to read and clearer.

Another example - a continuation dialog.

Simplify wording.

Again, let's identify what's going on and label the buttons for the actions.

A definite improvement.

Here's a final thought. Avoid negatives, especially double negatives. Some native English speakers find double negatives tricky, and many who learn English as a second language find them confusing (especially if their native tongue uses double-negatives for emphasis instead of inversion).