Just a short lesson in the English language as it is used in text (writing). When the following word begins with a vowel (a,e,i,o,u and sometimes y) you should use the word "an" ie. I think I will eat an apple.
When the following word begins with a consenant (all the rest of the letters of the alphabet) you should use the word "a" ie. I think I will climb a tree.
The English language is a very difficult one to learn and I applaud all of those who are attempting to use it when it is not the native tongue.
Good luck