glitching out 6s

anyone that can help me with this problem i bought an iphone 6s from straight talk a year ago from the website and since i got it, it has been glitching out on me going to different apps then i wanted and when i try to text someone it would type for me. i have restarted the phone and still does it. its up to date and idk if its because it came from straight talk and not the actual apple site but i would love to get some help with this **** phone its to the point where i wanna throw it out the window