Adding the Particle 가: Since both 저 and 나 end in a vowel (with no 받침), we can happily use 가 for both. (For a noun with 받침, we use 이. 가 and 이 have the same function. Which one you use simply depends on whether or not a noun ends in 받침. This is the same concept as using "a/an" in English depending on if a noun starts with a vowel or consonant sound.) 저+가 -> 제가 나+가 -> 내가 (BONUS: 너+가 -> 네가)(pronounced 니가) 제가, 내가, and 네가 are basically contractions of 저가, 나가, and 너가. However, you will never see the original (latter) forms. (Unlike in English when you can use either version of a contraction: "don't" or "do not" both sound okay, right) The meaning of 제가 is still "I/me" (humble). The meaning of 내가 is still "I/me" (regular/lower). The meaning of 네가 is still "you" (regular/lower). To understand the nuance, you must study and practice 이/가. Adding the Particle 는: Since both 저 and 나 end in a vowel (with no 받침), we can happily use 는 for both. (For a noun with 받침, we use 은. 는 and 가 have the same function. Which one you use simply depends on whether or not a noun ends in 받침. This is the same concept as using "a/an" in English depending on if a noun starts with a vowel or consonant sound.) 저+는 -> 저는 -> 전 나+는 -> 나는 -> 난 (BONUS: 너+는 -> 너는 -> 넌) The last arrow shows the contracted form. Both the full form and contraction can be used naturally, although, since it's humble, I see the contracted form of 저는 (전) less often than the contracted form of the casual 나는 (난) and 너는 (넌). The meaning of 저는/전 is still "I/me" (humble). The meaning of 나는/난 is still "I/me" (regular/lower). The meaning of 너는/넌 is still "you" (regular/lower). To understand the nuance, you must study and practice 은/는.

어디서 = 어디 (where or somewhere) + 에서 (dynamic place or source of action)

As i am also a student of this language, i will answer this to my ability. 어디가 is using where as a subject a sentence, 가 as a subject marking particles 어디서 mean where were you? 서 is usually used for past. 저는 is used to make l(저) as the topic of a sentence by using 는as topic particles and 제가 used to make me(제) as the subject of a sentence with the use of 가.