let me know if you need more specific whuinfo.
Why not include specific info with your initial query here? We don't have crystal balls that actually work, although I shine mine up once in awhile.
There are a HUGE number of Hall-effect sensors available... pick one and tell us which one it is. Better yet, include a datasheet or a link to a datasheet. Same-o, same-o for whatever "linear solenoid" you plan to use... like how much current will it draw at 12 VDC? Or do you know the coil resistance? What is the duty cycle? Will the solenoid be "on" all the time after the Hall-effect sensor actuates it? If not, how do you turn the solenoid off?
You can purchase Hall-effect sensors that interface directly to DC solid-state switches.You just have to provide a little voltage to the circuit, but we need to know what those two devices
are before we can suggest how to do that.
Why guess about what components you need? We usually give good advice here if you tell us up front WTF you are trying to DO instead of "guessing" how to do it. Or, you can grab a handful of transistors, resistors, and other components and have a go at it yourself. Make sure the color-code on the resistors is pointing in the right direction.