Bay`ah, in Tareeqah, is the act of pledging allegiance to the Shaykh, and being initiated into that Silsila. Doing so means you now become part of an unbroken spiritual chain that goes from your Shaykh to the Holy Messenger 
.
Bay`ah literally means a transaction, in this case it is the transaction of one’s loyalty. This idea of bay`ah sees its roots in the Qur’an and Sunnah. When RasoolAllah would successfully invite people to Islam they would give bay`ah upon the hands of RasoolAllah . This typically included the Shahadah (testimony of faith) and a few prayers of repentance. It also acted as a formal procedure which demonstrated support for RasoolAllah .
In the Qur’an, Surah Fath, Ayah 18, Allah ﷻ says:
“Certainly Allah ﷻ was well pleased with the believers when they swore allegiance to you under the tree, and He ﷻ knew what was in their hearts, so He ﷻ sent down tranquillity on them and rewarded them with a near victory.”
This ayah was revealed when the companions of RasoolAllah gave their bay`ah in support of RasoolAllah after hearing the news of the rumoured death of Sayyiduna Uthman 
. This ayah not only demonstrates that bay`ah is a praiseworthy act in the view of Allah ﷻ, but it also shows how doing so results in Allah’s ﷻ favours upon that person.
When we give bay`ah, we renew our Shahadah, and make promises to pray our Salah, give our Zakah, to perform Hajj, and to fast in Ramadan. We make Tawbah (repentance) for our previous sins, and are then initiated into the Silsila. After this, one becomes a mureed of that Shaykh. After giving bay`ah, we are to act upon the teachings of the Shaykh. They will prescribe a Wird (litany) that should be completed daily, there will be other forms of Dhikr that one should learn too.
Learning about the Tareeqah, the previous masters, Tasawwuf in general, and the Fard ul `ayn should be a priority as soon as one has taken bay`ah. Having made promises on the five pillars not only means to act upon them, but to learn the intricacies of them too. This shouldn’t be neglected as learning Shari`ah comes hand in hand with progression in Tareeqah.
The purpose in taking bay`ah is to begin the path of Tazkiya (self purification). The idea of Tasawwuf is to rid the heart of its diseases and to progress in terms of gaining closeness of Allah ﷻ. Taking bay`ah is the first step in this, and to be sincere upon the teachings of the Shaykh is the next step.
It is worth mentioning in regards to the saying;
“Whoever has no Shaykh, his Shaykh is the devil.”
This is not to be misunderstood; this does not mean if you don’t give bay`ah to a Shaykh, your guide is Shaytaan. It means that if you have no guide at all, be it a teacher of the Deen, a Shaykh, an elder who teaches you etc then you are susceptible to your own misinterpretations of Islam. This is why it is important to have an understanding of Deen, both in terms of Shari`ah, and Tareeqah. Knowledge of Tareeqah is only gained through company of the Shaykh. This is called Suhbah, or companionship, and is massively important in the Naqshbandi Tareeqah, as it said by Shaykh Baha’uddin Naqshband Bukhari 
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“Our way is of companionship, and the goodness is found within the gathering.”
After giving bay`ah, make it an effort to excel in what you are good at and to avoid that which is bad for you. Stick to the teachings of the Shaykh, keep attending the gatherings of Dhikr, make it a habit to recite the Awrad set for you, and most importantly, don’t abandon that which is Fard upon you.