Newbie player, currently have a fender mustang II, whats a good upgrade?

Been playing for about a year. I have a mustang II my sister gave to me that was sitting in her boyfriends garage collecting dust for however many years. Ive never really liked it, maybe im using it wrong but no matter what it just sounds bad. I get constant high pitched feedback and if i try adding any effects besides a clean sound it just gets super distorted and messy.

What would be a decent upgrade? Or if im perhaps using the amp wrong how do I use it properly.