I'm studying about the central force and Kepler's law, and I was wondering: by the first law, the trajectories of planets suppose to be elliptical. but from what I understood, most physical trajectories are almost circular in practice. so why does this happen?
Circles often are seen as a symbolism of stability and symmetry. A lot of physics simplified models use to consider circles as the trajectory of some motions because it's way simpler to work with them. Constant radius, which makes the measures involved with the inverse square laws (really present in nature) the same for every point around the circle. There are simply a lot of benefits of using this circle / spherical symmetry.
Unfortunately, nature is often not that simple. Kepler tried to be the most truthful possible to what concerns the trajectory of the planets and it turns out that the trajectory described by the planets was actually elliptic and not circular. Even though, there are still some problems that consider the orbit of some arbitrary planet as a circle, in order to make the calculations easier.
Be prepared. Throughout your education, in physics, you will find motions getting more and more complicated as you start to consider other factors that may influence your motion.