Salam Alikuumi have 2 questions1) how i must dress to a funeral?2) what can i say for my mom at the funeral? (which du'a? )

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. {C}

There are no specific clothes one is obliged or recommended to wear for the funerals. What is spread in some societies or amongst some people that they should wear black color clothes has no foundation in the Sharee’ah.

As regards the supplication in a funeral, then one should supplicate for the dead with mercy and forgiveness, and it is better to supplicate using the wordings that were reported from the Prophet in a funeral.

For instance, ‘Awf ibn Maalik narrated: "The Prophet offered the funeral prayer, and I memorized from him this supplication (that he said for the deceased), 'O Allah! Forgive him and have mercy on him. Grant him ease and respite. Make his resting place a noble one, and facilitate his entry. Wash him with the most pure and clean water, snow and hail. Purify him from sins as a white garment is cleansed of dirt. Give him in exchange a home better than his home (on earth) and a family better than his family. Admit him into Paradise and protect him from the trials of the grave and the torture of Hellfire.'" [Muslim]

Among the (Prophetic) supplications as well is that Abu Hurayrah narrated: "When the Messenger of Allah prayed a funeral prayer, he would say, "O Allah! Forgive those of us who are alive, and those who are dead; those who are present and those who are absent; the young and the old; males and females. O Allah! Let those of us to whom You have given life, live upon Islam, and let those of us whom You take (put to death) die in a state of faith. O Allah! Do not deprive us of the reward for patiently persevering his loss (and the reward for all acts of worship related to his death; prayer, supplication etc.), and cause us not to go astray after him." [Muslim, Abu Daawood, At-Tirmithi and Ibn Maajah]

You should know, dear brother, that being kind and dutiful to the mother does not stop by her death; rather, it continues after her death.

One of the ways of being kind and dutiful to your mother after her death is what the Prophet advised us to do, by paying off her debts, if she has any debts towards Allah or towards His Slaves, supplicating for her, giving charity on her behalf (i.e. while intending the reward for her), and keeping ties with her kinship.

Abu Usayd Maalik ibn Rabee'ah As-Saa'idi narrated: "While we were with the Prophet a man from the tribe of Salamah came to Him and said: 'O Messenger of Allah! Is there any kindness left that I can do to my parents after their death?' He replied: 'Yes, you can supplicate for them (and pray the funeral prayer on them); seek forgiveness for them, fulfill their will (or contract) after their death, maintain ties with those whom only kindness to your parents drives you to maintain, and honor their friends.'" [Ahmad, Abu Daawood, and Ibn Maajah. Al-Iraaqi classified it as Hasan in Takhreej Al-Ihyaa’]

For more benefit, please refer to Fataawa 245731 and 277330.

Allah knows best.