Dua to seek guidance (Istikhara),

اللَّهُـمَّ إِنِّـي أَسْتَخِيـرُكَ بِعِلْمِكَ، وَأسْتَقْـدِرُكَ بِقُـدْرَتِـكَ، وَأَسْـألُـكَ مِنْ فَضْـلِكَ العَظِـيمِ، فَإِنَّـكَ تَقْـدِرُ وَلاَ أَقْـدِرُ، وَتَـعْلَـمُ وَلاَ أَعْلَـمُ، وَأَنْـتَ عَلاَّمُ الغُـيُوبِ، اللَّهُـمَّ إِنْ كُنْـتَ تَعْـلَمُ أَنَّ هَـذَا الأَمْـرَ (وَيُسَـمِّي حَاجَتَـهُ) خَـيْرٌ لِي فِي دِينِـي وَمَعَـاشِي وَعَاقِـبَةِ أَمْـرِي، فَاقْـدُرْهُ لِي وَيَسِّـرْهُ لِي ثُـمَّ بَارِكْ لِي فِيـهِ، وَإِنْ كُنْـتَ تَعْـلَمُ أَنَّ هَـذَا الأَمْـرَ شَـرٌ لِي فِي دِينِـي وَمَعَـاشِي وَعَاقِـبَةِ أَمْـرِي، فَاصْرِفْهُ عَنِّي وَاصْرِفْنِي عَنْهُ وَاقْـدُرْ لِي الخَـيْرَ حَيْـثُ كَانَ ثُـمَّ أَرْضِـنِي بِـهِ.

Allâhumma innî astakhîruka bi-‘ilmika wa astaqdiruka bi-qudratika, wa as’aluka min fadlika-l-'azîmi. Fa innaka taqdiru wa lâ aqdiru, wa ta’lamu wa lâ a’lamu, wa anta ‘allâm-l-ghuyûbi. Allâhumma in kunta ta’lamu anna hâdhâ-l-amra khayran lî fî dînî wa ma'âshî, wa 'âqibati amrî fa-qdurhu lî, thumma bârik lî fîhi. Wa in kunta ta’lamu anna hâdhâ-l-amra sharrun lî fî dînî wa ma'âshî, wa 'âqibati amrî fa-srifhu 'annî, wa srifnî 'anhu, wa qdur lî-l-khayra haythu kâna thumma arddinî bihi.

O Allah, I seek Your counsel by Your knowledge and by Your power I seek strength and I ask You from Your immense favour, for verily You are able while I am not and verily You know while I do not and You are the Knower of the unseen. O Allah, if You know this affair -and here he mentions his need- to be good for me in relation to my religion, my life, and end, then decree and facilitate it for me, and bless me with it, and if You know this affair to be ill for me towards my religion, my life, and end, then remove it from me and remove me from it, and decree for me what is good wherever it be and make me satisfied with such.

Jaber Ibn 'Abdullah, May Allah be pleased with him, narrated that the Prophet (salAllahu alayhi wa'sallam) said: if any of you intends to undertake a matter he should offer a voluntary two rak'a prayer and say [this].

Sahih Bukhari: Vol 2, Hadith 263; Vol 9, Hadith 487.

There are different ways to do this dua, this is the most basic way to perform it. Many perform this for matters of marriage, to ask if the union will be sound and accepted by Allah (Subhanahu Wa Ta'ala).