Before I list the minutes, pay attention to the endings of each minute.  I’ll explain why later:

1 : いっぷん

2 : にふん

3 : さんぷん

4 : よんぷん

5 : ごふん

6 : ろっぷん

7 : ななふん

8 : はっぷん or はちふん

9 : きゅうふん

10 : じゅっぷん

If you look carefully at the endings, you’ll see that each of the minutes end in either ぷん or ふん.  These endings will hold true when you get to the higher numbers.

For example: 11 minutes in Japanese has the same ending for 1 minute in Japanese.  1 minute = いっぷん.  Notice how it has the ぷん ending.  Take the ぷん ending and attach it to the word for 11, which is じゅういっぷん or 11 minutes.

The same goes for 18.  Look at the second digit for 18.  It ends in an 8.  Look at the ending for the number 8, which is either ぷん or ふん.  Just attach either ぷん or ふん to the number 18 and you’ll get 18 minutes.

Now let’s look at how to form 20 minutes.  The second digit ends in zero, which means we’ll have to look at the ending for 10 minutes.  The ending for 10 minutes end in ぷん.  That means that we’ll have to add ぷん to the number 20 to get 20 minutes.  The same goes for 30, 40, 50 and onwards.

Hopefully this method will help you with the minutes!