How to balance a reaction in chemistry?

Do you mean a reaction equation? If yes then I will explain how to balance a chemical reaction equation.

Then the answer my friend is to make sure the number of elements on either sides of the reaction equation are the same.

Say for example, the reaction:
Na + Cl => NaCl (formation of common table salt)

1. You look at the elements separately;
Na (sodium) and Cl (chloride)

2.Then you list down the number of the elements on one side first (best to work with the right side first). Now the number of the elements will always appear AFTER each element or compound in subscript (unless the reaction has already been balanced then the no. comes BEFORE an element or compound) - and as always if the no. of elements is 1 there is no need to write down 1, therefore in the reaction above there is no number written so you know the number of both Na and Cl is 1.
Na = 1
Cl = 1

3. Check to see if the no. of elements on the left side are the same;
Yes, they are the same because 1 sodium (Na) on the left side and 1 sodium (Na) on the right side as well, also 1 chloride (Cl) on the left side and 1 chloride on the right side as well.

4. If the no. of element on the right side of the reaction are the same as on the left side then your reaction equation is balanced.

5.If the no. of elements on the right side are not the same as on the left side then insert numbers before the elements on the left side to balance them out.

Let's look at another example. Say:
O2 + H2 => H2O (formation of water)
ps. you should know that the numbers written after the elements like in this example should be in subscript, but the typing format of this website just won't allow it so please excuse the typing error. Also, like as mentioned in step 2 above, the numbers in this example are written after an element to indicate the reaction has not been balanced.

So let's follow our previous steps;
1. O2 (oxygen) and H2 (hydrogen)
ps. again note that the no. after these elements should be in subscript

2. (on the left side first) O (oxygen) = 2 and H (hydrogen) = 2

3.No, the number of elements on either sides are not the same. Because there are 2 oxygen atoms on the left side but there is only one on the right side.
left side: O (oxygen) = 2 and H (hydrogen) = 2
right side: O (oxygen) = 1 and H (hydrogen) = 2

4. The reaction is not balanced

5.original reaction equation:
O2 + H2 => H2O
balanced reaction equation:
O2 + 2H2 => 2H2O

Now we have 2 oxygen on both left and right side and 4 hydrogen on both left and right side as well. So the reaction is now balanced.
left side: O (oxygen) = 2 and H (hydrogen) = 4
right side: O (oxygen) = 2 and H (hydrogen) = 4

Note: when balancing a reaction equation, you only insert numbers before an element NOT after.