According to what in scripture, we should do Taqlid (follow a Mujtahid) in religious problems?
In Surah Nahl, verse 43 we read:
“Ask those who know if you don’t know (something).”
let me give you an example:
What do you think an engineer would do if he got sick?
start spending so much time studying and doing lots of researches to figure out how he can get rid of the disease he's suffering from.
go to a doctor and ask the doctor to save him the trouble of all those exhausting effort.
A sane person will definitely choose the second way.
The above-mentioned verse is exactly implying an intellectual rule which is: "a non-expert person in a certain field has to go to an expert in that field."
Knowing that the skill of distinguishing between authentic Hadiths and the fake ones, understanding the correct meaning of the verses and traditions and reconciling between them when coming across a contradiction is not reachable without years and years of hard work, we can simply conclude that the only reasonable way of finding religious rules and regulations for non-expert people is following Islamic experts (Mujtahid).
Well more than religious scripture, its common sense that tells us to do Taqleed.
For example, if you get sick you go to a doctor to find the cure for your disease.
Similarly as times change a new things come about, we must ask a scholar for guidance towards the right and away from the wrong. For example, now a days many food companies use ANIMAL ENZYMES in their food.
At the time of the prophet there was no such thing as animal enzymes being used in food, so how do we know if this is hall or haram?? ... When we do taqleed of a scholar we can know by the fatwa he gives.
In the Shia view is that Allah had selected a successor of The Prophet Muhammad, and that was Imam Ali. So the successor was to be the true godly appointed Caliph/Wali/Imam (Wilayat). And that Godly appointed Caliph/Imam/Wali was the person who everyone has to do taqleed of.
After the occultation of the 12th Imam (Imam Mahdi), the concept of taqleed became general. That now the representatives of the Imam (Scholars and Mujtahedeen) were able to give fatwas according to the knowledge they acquired.
So the scholars are the channels of Wilayat that go towards the Imam and the Imam is the channel of Wilayat that goes to the Prophet and the Prophet is the Channel of Wilayat that goes towards Allah (SWT).