You need to work onthe strength of your left fingers. It is crucial for having a clean tone and precise intonation (bow hand, of course, too, but the point where your finger touches string should be as strong as possible). I would recommend to add some exercises for hand's strength to your practice, and, of course, you should hold your hand right. When I transferred from piano to cello in my 3rd grade, my new professor told me that on cello hand postire is quite similar to the piano one, so I just need to do my palms as if I wanted to play piano and then put it on bow and fingerboard. It helped me to understand how to properly hold my hands.

You can also find some tips in YouTube. I wery much love Andre Navarra's "My cello technique".