To be able to go sideways , you'll have to modifiy your bike a bit more than just getting the tires right. We recommend to get more steering angle. Tip: shorten the steering stoppers. In addition, lower and stifen the bike suspension in the front and rear. You want to get the center of gravity as low as possible. We also recommend a flat track handle bar for better control.
We recommend to have proper riding gear. Either the motocross style: motorcross boots, helmet, and gloves, or the proper motorcycle gear: full face helmet, leathers, and boots. BUT most important: a steel shoe for your left boot as shown in the picture.
1. get a set of 19" rims for front and back, with 19" tires (such as the Maxxis DT-3).
2. remove the front brake and pick up mount, you'll never need it again
3. street bike front forks (i.e.: Yamaha R6 or Honda CBRR 600)
4. put in a stronger spring for the rear suspension and lower the bike
5. flat track triple clamp (i.e.: A&A racing) and flat track handle bar
6. move the rear further inwards
7. get more torque out of your engine such as a big bore kit
Ready, to hit the dirt!