Assuming your taking Fundamentals 1, I'd review with Lisa or Chris proper form. You can post directly to them in their respective subforums. Home page, scroll down and you'll see their names. However, I see Chris did answer you already on a similar question previously so to reiterate what he said it comes down to direct placement of your fingertip on the string. I remember Lisa using an analogy of to pretend your fingertip is a Ballerina foot. In Fundamentals 1 there is a point where she goes over strategy on how to troubleshoot these errors so continue practicing and going forward with lessons if you haven't reached that part or go over it again.

One thing that could also be happening is you don't have enough callous buildup on your fingertips. Your probably pressing down too hard to get clear tone and the finger sort of widens out at the tip and the fleshy part is touching the adjacent strings. These are all beginner problems we all went through so practice and repetition will fix all of this. Once you get a good hard callous and learn to press the string just behind the fret you'll realise you don't need to press too hard and you'll get a cleaner tone.

Hope this helps.

Moe