In tennis you don't really aim in the conventional sense- just as in tossing a basketball-- most of it is practic, experience and feel.
But if you insist on thinking in terms of "aiming", consider archery. Some archers use a small pin that sticks into a rubbery strip in front of their bow.
At first they put the pin (think forward sight) just anywhere based on experience-- then they shoot a few arrows. If the arrow group two inches up and to the right, then the archer adjusts the pin and tries again until eventually he finds the right spot and hits dead-on every time.
Something similar can be done in tennis, but without the sights.
Simply "aim" for a specific spot- and then take note of where your ball landed, if it was long or wide, or both, then adjust and try again until, eventually, you are striking the the intended spot nearly every time.
On the other hand, the very fact that you ask this question, and the down-the line one, also-- well, I have the feeling that your time and energy can be better spent on the basics.