All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Among the etiquette of supplication is praising Allah, The Exalted, and invoking blessings on the Prophet before reciting the supplication. It was narrated on the authority of Fadhaalah ibn ‘Ubayd that the Messenger of Allah heard a person reciting a supplication during the prayer, where he did not glorify Allah, The Exalted nor invoke blessings on the Prophet . Thereupon, the Messenger of Allah said: “This one is hasty.” He then called him and said to him – or to another person: “When any of you prays (and supplicates in the prayer), let him start with glorifying and praising his Lord and then invoke blessings on the Prophet; thereafter, he should supplicate Allah for whatever he wishes.” [Abu Daawood, Al-Albaani: Saheeh (authentic)]
Al-Munaawi said in Faydh Al-Qadeer: “The Hadeeth reads: ‘When any of you prays,’ the funeral prayer aside, ‘let him start with praising Allah, The Exalted.’ Another version reads, ‘Let him start with praising his Lord,’ meaning to praise and glorify Him with beautiful words of glorification and praise, exalting Him and expressing gratitude to Him, repeatedly. Praise means doing what reflects exaltation and glorification. The Hadeeth reads, ‘Then invoke blessings on the Prophet,’ meaning to conclude the Tashahhud with invoking blessings on him; ‘thereafter let him supplicate Allah for anything he wishes,’ meaning that it is recommended to supplicate Allah, The Exalted, after reciting what was mentioned above (praise of Allah and invocation of blessings on the Prophet) for whatever is permissible for him to ask for with regard to his religion or worldly life.” [End of quote]
Accordingly, saying “Subhanallah Walhamdulillah Wala Ilaha Illa Allah Wallahu Akbar (Glory be to Allah, and all praise be to Allah. None is worthy of worship but Allah, and Allah is Great),” before the supplication is a form of praising Allah and glorifying Him.
It is permissible for you to praise Allah, The Exalted, and supplicate Him in any language that you know.
According to the Fatwa of the Permanent Committee (for Islamic Research and Fataawa), “It is permissible for a person to supplicate Allah, The Exalted, in any language that he knows, be it Arabic, English, Urdu, or any other language.” [End of quote]
You asked, “I heard about the Isme A‘tham Du‘aa (supplication of the greatest Name of Allah), what is its wording?” We do not know of a supplication that is called Isme A‘tham Du‘aa. However, if you are asking about the greatest Name of Allah, by which, if He is invoked, He responds (to supplication), the scholars held different opinions in this regard, the strongest of which are two: ‘Allah’ and ‘Al-Hayy Al-Qayyoom’ (The Ever-Living, The Sustainer of [all] existence). Ash-Shirbeeni, may Allaah have mercy upon him, said in Al-Iqnaa‘: “According to the researching scholars, ‘Allah’ is the greatest Name of Allah, and it was mentioned in the Quran in two thousand three hundred and sixty places. Following the opinion held by a group of scholars, An-Nawawi held that the greatest Name of Allah is ‘Al-Hayy Al-Qayyoom’.” [End of quote]
It is not obligatory on you to invoke blessings on the Prophet after supplicating Allah by His greatest Name. However, among the etiquette of supplication is to invoke blessings on the Prophet before making the supplication as mentioned above.
Allah Knows best.