555 Clock Circuit - how to choose resistor value?
Good data sheets for 555 timers, like this one, from ON will have a schematic and an equation or a graph (or both) showing resistor/capacitor combinations.

Note that when you get much above 100kHz you need to be careful: there's a lot of different ICs out there called a '555 timer'; some are good old bipolar circuitry (like my linked data sheet), some are CMOS, some are fast, some are low-power (and therefore slow).  If you are outside of that magic 100kHz mark, reliably, you probably can't use just any old 555 -- you need to use one that's actually capable of going that fast.