How can I write out 1.5?

You probably don't want to do it at all. Noninteger numbers are best written in their decimal form (1.5, 5.0). If they can be expressed as simple fractions, you can do that instead: “one and a half” for 1.5.

If you absolutely want to write it out, then you'll have to do it the way they are spelt: “one point five”, “five point zero”. You can use oh instead of zero (“five point oh”, “oh point five”) to reflect an informal spoken style.

There are two main ways to express decimal numbers in words.

Reading the decimal. In this method you read the part of the number left of the decimal point as a normal integer, then the word “point”, followed by the numbers to the right of the decimal point. They are read either as individual digits, or sometimes in pairs as two-digit numbers. Examples:

• 1.5 = one point five
• 5.0 = five point zero or five point oh
• 1.05 = one point zero five or one point oh five
• 1.21 = one point two one or one point twenty-one
• 17.365 = seventeen point three six five or seventeen point three sixty-five
• 3.9715 = three point nine seven one five or three point ninety-seven fifteen

This method of reading out decimal numbers is the most common these days, and would be how how almost anyone would read out a decimal number when speaking out loud.

Converting to a decimal fraction. In this method, the part to the right of the decimal point is treated as a fraction whose numerator is an integer represented by the digits to the right of the decimal point and whose denominator is a power of ten, depending on the number of digits:

digits  denominator
     1  tenths
     2  hundredths or one-hundredths
     3  thousandths or one-thousandths
     4  ten-thousandths
     5  hundred-thousandths or one-hundred-thousandths
     6  millionths

Examples:

• 1.5 = one and five tenths
• 5.0 = five and zero tenths
• 1.05 = one and five hundredths or one and five one-hundredths
• 1.21 = one and twenty-one hundredths or one and twenty-one one-hundredths
• 17.365 = seventeen and three hundred sixty-five thousandths or seventeen and three hundred sixty-five one-thousandths
• 3.9715 = three and nine thousand seven hundred fifteen ten-thousandths

This method is a pretty unusual and old-fashioned way of reading out decimal numbers, although it is unimpeachably correct for the standpoint of historical tradition. You would generally only find this method of reading or spelling out in very formal situations or where a very strict and old-fashioned style guide is in place.

I'd like to add a third option to @nohat's answer. In scientific settings you can often eliminate the decimal altogether by changing the units. For example, if I was reading this:

We found that there was a .25 meter separation between component abc and component xyz.

I would probably read this as "We found that there was a twenty five centimeter separation between ..."

Putting everything into the base SI units is good for writing academic papers because it prevents someone from misleading with different units. (E.g. 700 Billion bailout, 180 Million bonuses) However, when speaking the less awkward pronunciation usually makes removing the decimal helpful for people to understand. People don't try to reconstruct long strings of digits in their head, their eyes simply glaze over and stop paying attention.

All the answers have been quite good. I suspect that this is determined locally, to some degree, a lot by context (are you rattling lots of figures off, or is this a formal presentation, etc.) and by the required formality / style guide.

I'll throw a few more into the mix (keep in mind my midwestern American English sense):

• 1.5 becomes one and one half or one and a half. More common: one point five. (.25 becomes quarter, .75_ becomes three quarters.)
• 5.0 becomes five, five-all, five-even, or five point oh and occasionally five point zero.
• 2.79 becomes two point seventy-nine or two point seven nine. Rarely any other way is used, but if I had to go "old fashioned", it'd be two and seventy-nine hundredths (as in writing checks). If your context has established an expectation of a decimal point, then you might hear two seventy-nine.
• $6.34 becomes six dollars and thirty-four cents in formal use, but often said six thirty-four unless the meaning would be overly ambiguous (e.g.: 100.14 would be said one hundred dollars and 14 cents, not one hundred and fourteen.)
• 11:46 becomes eleven forty-six almost all the time. Only when there are no minutes to report does "o'clock" occasionally get added. Occasionally half-past (:30), five till (:55).

Side note: Occasionally nought and ought are heard for zero. It raises more eyebrows and question-marks when it is used, though, so best to avoid it.

Writing a Decimal in Words:

1. Look to see if there is a number to the left of the decimal; if so write it out. If there is no number to the left of the decimal, skip to step 3.
2. Write an and for the decimal point.
3. Write out the number to the right of the decimal. Do not yet include the place value.
4. Determine the place value of the last digit to the right of the decimal. Write the place value.

So, for "1.5" you should write "one and five tenths" and for "5.0" the term "five and zero tenths" would be correct.

P.S: I don't know what is more formal or informal in written-English, what I've mentioned is just the way to write out a decimal.