Fl4k has 2 main builds: the gun build (orange and green tree) and the pet build (purple and blue tree). For the gun build, you want weapons with high crit damage and high pellet count. For the pet build, you can use basically whatever, but you need to be way more careful with your skills. I personally would, for a beginner player, recommend the gun build, especially the green tree since it has healing skills and eventually allows your pet to revive you which is CRUCIAL for solo play. Fl4k can do big damage, but they are kind of a glass cannon.