One Tomoe
It is only first unlocked under some stressful or emotional condition after which the Sharingan may be used freely. When the Sharingan is first activated by a user, it has a different number of tomoe around the central pupil. The user's pupil turns bloody red, and gains an extra pupil, or tomoe within each eye that will surround the original, centered pupil. When active the user now has the ability to track speeds that normally cannot be seen by the regular human eye. They also gain the ability to see through visual D rank genjutsu and can predict most taijutsu and ninjutsu movement and technique, allowing one to be able to react best to the situation.
Basically by gaining it through stressful or emotional conditions means...you need to get the living hell beat out of you
Two Tomoe
The wielder of the Sharingan, through continued use and training, changes its form further, which involves gaining one more tomoe. It allows the user to perceive motions as well as be able to copy the motions perceived. When activated the user's eyes turn blood red, and gain two extra pupils, the tomoe which surrounds the original pupil. At this level the user gains a better understanding of the sharingan. The user can now anticipate and analyze faster movement, as well as mimic minor movements made by an opponent. The user can now track handseals as well, allowing one to predict ninjutsu and help avoid it. The user is now able to pierce through visual C rank genjutsu and below.
To summarize
1t is unlocked by near death experience. Once unlocked you can see through d rank genjutsu and can begin to read handseals.
2t is unlocked through intense training. Once unlocked you can see through c rank genjutsu and you can perceive motions
And np id mind waiting