1. Difference between ある and いる
Both ある and いる translate to "exist". The main difference is that ある is used for inanimate objects and いる is used for alive/animate things.
So if you want to say "there is a cat", you would say "ねこ が いる". But if you wanted to say "there is a chair", you would say "いす が ある".
2. The sentence you mentioned: いま どこ に いるの
2.1. To understand this, let's first analyse the phrase without the の in the end
いま どこ に いる
The translation you pointed out, "Where are you now?", is correct. Why?
- いま means now
- どこ means where
- いる means to exist;
- に is a particle that, when used alongside いる, means the location of the object.
Therefore, this sentence would translate roughly to "Now where do [you] exist?". Note that the "you" is guessed. It could very well be "we", or "him", and such.
That rough translation is the same as "Where are you now?" in better english.
Learn more about the に particle here
2.2. Understanding the の in the end
Don't worry if you get confused with the の particle. It has various meanings, so you have to detect what is its role in each situation. Sometimes it will mean possession, like in "Bob の ほん" (Bob's book). In the end of sentences, though, it will in general consist of the explanatory の particle. It justs add an explanatory tone to the sentence. The sentence you mentioned is a question, so the の particle in the end emphasizes that the questioner is looking for an explanation.
Learn more about the explanatory の particle here
3. たべたい ひと が いる
You are correct to guess the translation: There is a person/There are people I want to eat.
EDIT: At a first glance this can be translated as There is a person/There are people I want to eat, but clearly this is a bit unusual, although still possible, if this is a fictional setting, and the 'thing' speaking is a monster, for example. But this could also be translated as There is a person/There are people who want(s) to eat.
(Thanks @Yosh for bringing this up)
4. Last words
Both links I suggested above are from the same guide, called Tae Kim's Guide to Learning Japanese. I totally recommend it, it is free, this is the link.