It is informal. I'm not sure what you're trying to say.
Here's a table showing the pronouns.
Genders: You have to learn the gender with the noun. If you mouse over the noun, you will get a definition and at the bottom, the gender. This doesn't always work as not all nouns have been updated, so you may have to look the gender up in a dictionary.
Why not "Ich sehe der Apfel"?
1) They asked for "You see the apple". Ich = I. du, Sie, ihr = you (informal singular), you (formal singular and plural), you (informal plural)
2) Apfel is a masculine noun in the accusative case (in this sentence). Der = masculine nominative (among others). Den = masculine accusative (among others).
Link to cases: http://coerll.utexas.edu/gg/gr/cas_01.html
Link to definite articles: http://en.wikipedia.org/wiki/German_declension#Definite_articles.5B1.5D
Masculine Feminine Neuter Plural
Nominative -er -e -es -e
Accusative -en -e -es -e
Genitive -es -er -es -er
Dative -em -er -em -en
The German articles are a pain to learn; this list is useful to keep in the back of your head when you have such a problem (definite articles). Also you need to master the use of German cases in order for everything to fit. http://german.about.com/library/blcase_sum.htm
I suggest you take a look over these grammar rules and then just carry on practicing; eventually it will just come natural.