Translation:Tell that girl that I am already going.
native Russian) first there's no word 'пойдёмся'.
The phrase "я уже иду" used in a situation when someone is expecting you to go somewhere, and she/he is asking you "Ну, ты уже идёшь?" (Well, are you going already?) You say "я уже иду" , which could mean that you are already on your way, but mostly it means that you are still getting ready (like putting on your clothes) and be on your way in a short while.
Yes, in this sentence and context, that usage is extremely awkward in English. Use of "already" and present progressive is not a usual way of saying things in English.
It is not wrong to use this format, it is just unusual, because "already" usually refers to a past even that has been completed: "Tell that girl that I have already gone" or "I already left".
But then, уже is used in Russian in ways that just don't fit "already" in English, and, while the translation might be the closest one can get to the meaning of the Russian sentence, it makes more sense in Russian than it does in English.
Have look at the link below for Determiners, to see a declension table for этот.
These are links to declension tables for a variety of Russian words. I keep them bookmarked in my browser and in a spreadsheet on my computer under separate tabs, so that I can easily navigate to what I need to look up.
Nouns & Russian Spelling Rules: https://forum.duolingo.com/comment/29038061
Personal Pronouns: https://forum.duolingo.com/comment/29119997
Non-Reflexive Possessive Pronouns https://forum.duolingo.com/comment/30122740
Reflexive Personal Pronouns Свой, Себя & Сам: https://forum.duolingo.com/comment/29965925
Prepositions, Case of Objects, and Meaning: https://forum.duolingo.com/comment/28544274
Determiners & Interrogatives: https://forum.duolingo.com/comment/30373766
Hi Ella - these charts should help - because the verb "tell" is dative https://www.pinterest.ru/pin/105342078765018327/?lp=true