If it were regular, yes :)
But this is one of a small group of verbs whose present tense comes from a past tense, and so there is no -t for "er, sie, es" nor an -e for "ich".
Compare English, where there is a similar small group of verbs: we say "he can, he may, he will", for example, and not "he cans, he mays, he wills" - those verbs have a similar origin in a past tense and thus also have no -s in the third person singular just as "he did, he said, he had" have no -s.
Fun fact: most such "preterite-present" verbs are modal verbs (in both English and German), but at least one is not: wissen "to know" is historically derived from the past tense of a verb meaning "to see" -- to know something is to have seen it :)
That's only of historical interest, but what learners do have to bear in mind is that it's ich weiß and er weiß, again without an ending in the present tense.
Because the apple is grammatically masculine.
einen is the masculine accusative form -- needed here because the apple is the direct object of the verb "want".
eine would be a feminine form. For example, you might say Er will eine Birne. "He wants a pear.", because pears are grammatically feminine.
- masculine nominative singular, e.g. Da ist ein Löffel.
- neuter nominative singular, e.g. Da ist ein Messer.
- neuter accusative singular, e.g. Ich sehe ein Messer.
and einen is:
- masculine accusative singular, e.g. Ich sehe einen Löffel.
So which one to choose depends on the gender of the noun and on the case that that noun is within a given sentence.
"a" and "an" are the same word in English -- there is no distinction in meaning. The choice is purely down to the sound of the next word. For example, "an elephant" but "a grey elephant", or "a door" but "an open door": the choice of a/an is not due to the "elephant" or "door", as it would be in German (ein Elefant, ein grauer Elefant; eine Tür, eine offene Tür).
As such, they both map onto the German indefinite article, which is ein or eine (or other endings depending on the gender and case).
It's not possible to say that English "a" always maps onto one German word and English "an" always onto another.
You choose the form of the verb that agrees with the subject.
In English, we say I want and he wants; we can't say I wants or he want.
So you have a look whether the German is ich will or er will and then choose the appropriate verb form in English depending on the subject.