can I attempt jee mains exam in only computer???

Hi candidate!
Jee main is conducted in online mode i.e. you have to appear the test at the alloted centre where computer device are well set for you.
You do not need to carry your own computer.
If , you are asking for the practice for jee main , then the NTA has its own app named NTA abhayas which you can install at your device and practice in the same format which occurred in exam. You can appear for these mock test at phone as well as computer. It will be good if you go through paper guidelines well before the exam and study about the different button to select , review and submit answer so that you don't face any problem during exam.
All the best!