When we add decimals, there are three basic principles that we need to remember:
 Write the numbers one below the other making sure that you line up the decimal points.
 Add zeros if necessary to get the same number of decimal places in each number. Then add as if the numbers were whole numbers.
 Place the decimal point in your answer so that it is lined up with the decimal points in the numbers added.
Examples:

3.04 + 14.3 
Explanation 

3.04
+14.30
17.34 

Write the numbers, making sure that you line up the decimals.
Add one zero to 14.3 so that both numbers have the same number of decimal places.
Add your numbers as if they were whole numbers.
Place your decimal so that it is lined up with the numbers added. 
Another way to remember where you put the decimal in an addition problem is to make sure your answer has the same number of decimal places as each of the numbers being added. In example 1, each number has two decimal places.
2. 
15.3 + .247 + 126.56 
Explanation: 

110100
15.300
.247
+ 126.560
142.107 

Write the numbers, making sure that you line up the decimals.
Add two zeroes to 15.3 and one zero to 126.56 so that each number has three decimal places.
Add the numbers as if they were whole numbers.
Place your decimal so that it is lined up with the numbers added. 
If you are adding a decimal number and a whole number, you need to understand that you can place a decimal to the right of the whole number to make it a decimal.
Look at these examples:
 25 becomes 25.0. then add zeroes so that each number that you are adding has the same number of decimal places.
 1 becomes 1. 0
Examples:
1. 
37 + 2.34 + 9.7 
Explanation: 

11000
37.00
2.34
+ 9.70
49.04 

Make 37 a decimal by adding a decimal point to the right of 37.
Add two zeroes to 37. And one zero to 9.7 so that each number has the same number of decimals.
Add the numbers as if they were whole numbers.
Place your decimal so that it is lined up with the numbers added. 
2. 
125 + .276 + 53.2 + 27 
Explanation: 

1100000
125.000
.276
53.200
+ 27.000
205.476 

Make 125 and 27 decimals by adding a decimal point to the right for 125. and 27.
Add three zeroes to 125., two zeroes to 53.2 and three zeroes to 27. so that each number has three decimal places.
Add the numbers as if they were whole numbers.
Place your decimal so that it is lined up with the numbers added. 