Here, try using a calculator like this one:
https://www.daycounter.com/Calculators/NE555-Calculator.phtml
Or you could just do the math yourself:
Frequency=1/T= 1.44/((R1+2*R2)*C)
Low Time= 0.693*R2*C
High Time= 0.693*(R1+R2)*C
Duty Cycle= (R1+R2)/(R1+2*R2)
EDIT: Just make sure you follow the schematic shown in the link if you plan to use that calculator. Also, if you decide to add capacitors, look at the datasheet for the 555 timer to see how they're supposed to be connected.