After taking your shahada first thing to do is purify himself by taking a complete bath (Ghusl) and then correcting the Faith.
in Islam, Faith stands on six pillars and first things first for a muslim is to believe in these six terms of Faith also a Muslim should not spend time without Ghusl.
Ghusl simply renews our state of purity in front of Allah. Ghusl is obligatory in certain events. If these events happen you need to take ghusl again.
Here is six pillars of faith a muslim must accept and believe in all of them.
Allah exists and is the only Creator, Sustainer, and Planner of everything in existence.
Mankind was created to dedicate their worship for Allah.
Allah is the only one who has right to be worshiped.
Angels exist, that they are the creation of Allah, and that some of them have been given names. (Jibreel and Mikael etc.)
The duty of angels is to serve and obey God in every command, and they never exceed God's laws and orders.
in Islam, it is an obligation to believe in all divine Messengers. Leaving off believing any Prophet or Messenger constitutes to disbelief in Islam.
A Muslim must believe in all the prophets sent, including those whose names have been reported to us and those whose names have not been reported to us.
There are 25 messengers who have been mentioned by name in the Qur’an and there are many not mentioned in Qur'an, a muslim must believe all of them without leaving even one.
In Islam, it is a part of faith to believe all the books and scriptures (original versions, not the versions existing today) that were revealed to the prophets.
Allah sent those Books to the followers of specific Prophets until those books are corrupted and modified by people.
Allah sent down the Quran as a last Revelation and Guidance for humanity.
Scriptures
A Muslim must believe that there is another life after death and that life after death is eternal.
Believing in the Akhirah (Afterlife) includes ;
Allah is free from time and Allah knows everything from eternity to eternity. Allah is Aware of all the good and bad that will happen.
Allah has given man a will and knows what man will do with his will. Allah does not give everything to mans will, somethings are already decided by him. Like who will be your parents, your height and appearance etc. Everything we do and we will do is already written because Allah is free from time and Allah knows everything.
Simple example of Destiny understanding in islam is, we cannot choose our mother and father, this is a destiny ordained by God. On the other hand doing or not doing something forbidden may depend on a person's will; God allows both situations to happen and He alrady knows whether the person will do that forbidden thing or not.
A new Muslim must begin to practice Islam as much as he can, after being purified by taking Ghusl and accepting the Islams Six Pillars of Faith.