In the name of Allah the most Beneficent and Merciful
Ijtihad can only be performed by those qualified to do so. The criteria when asked to Imam Ahmad Ibn Hambal (May Allah shower His mercy upon him) said that one needs to know at least 400,000 Hadith by heart and their Isnad (Chain of narrations). As you will probably agree there is no one capable of that any more. Even the great Hadith Master Imam Suyuti was not regarded as a Mujtahid by scholars of his time. However, when Imam Mehdi comes he will be a Mujtahid Imam and hence will use his own Ijtihad.
The solution is to follow a learned Muqallid (follower of one of the four Imams).
Of course Allah knows best.
[Answer provided by: Muhammad Salim Ghisa]