An attenuator with a DI line out. Resistive loads (hotplate, Jett attenuator, etc...) will be cheaper, but a reactive load (two notes, Suhr reactive load) will sound more real. Then something like Fractal Audio cab lab or two notes Wall of Sound to run Cab sims on your Computer. If you don't have a DAW software, Cab Lab has a standalone version. You will need an audio interface that has a HighZ (aka instrument in) to take the signal into the computer.