I'm currently only watching the anime. I hardly read the manga but I'm very curious as to how humans become titans. Is it by some kind of drug similar to the one Eren was injected with by his father?
This contains major unmarked spoilers if you've only watched the anime, as will the linked questions.
In Eren's case, it's through a particular serum that his father injected into him just after the fall of Shiganshina, shown in chapter 71 of the manga. This serum was stolen from the true royal family by Eren's father, and is used to allow the subject to turn into a titan. As well as Eren, Rod Reiss is also shown as being able to transform after he has some of the serum:
"...he himself scrounges the fluid off the floor[...]. After ingesting it, he turns into a gigantic Titan considered by Levi and Armin to be bigger than the Colossal Titan."
Rod Reiss on the SnK Wiki
Certain humans that are also 'titan shifters' seem to have this ability inherently, or at the very least, it hasn't been covered as to where they gained it - this includes the likes of Annie, Reiner and Bertholt.
Additionally, if a titan shifter is eaten, a titan with a human inside will gain the ability to shift between two forms. This is evident in the case of Ymir, discussed in this question.