How to stop my baby from thumbsucking? ?

Hi Maricor, It would be a time taking process and you perhaps can begin by keeping your baby engaged in things. If he a few months old, then all you can do is, keep taking his thumb out of his mouth whenever he puts it in. If he is about an year or so, you can tell him no repeatedly when he does that. You can may be apply some cream on his thumb, so that he doesn't put it in his mouth. You can clap for him when he takes out his thumb form his mouth. This will make him feel good and he would do it often for your appreciation. You can also give him a little treat for not doing so.