It would be helpful if we knew why you were losing.
Stay close so she can't get stage control with missiles for free, stay out of ftilt range, be ready to follow up any of her wall out moves ending lag with (this is important) drill, running shine, or a grab (if she's conditioned to shield). If you hit her shield, just back off. Just work on nailing the simple punishes that lead into upsmash.
Most Samuses are happy to play in the corner, but most of your offense is going to be earned in small portions when they're here and you're applying good corner pressure. If they're not in the corner, be aware of wavedash back punishes because Samus has some mean tools.
Samus is possibly Fox's most disciplined matchup, so be careful with it.