The rocket equation is going to be your friend here. In stock, a perfect and i mean perfect launch trajectory with perfect thrust to weight ratios to avoid drag loss while minimizing gravity losses simultaneously, orbit can be achieved with 3300 m/s to 3400 m/s of delta v (change in velocity) 3600 m/s is a good number to shoot for with excess just in case. The rocket equation is what you use to determine how much your ship has.
Change in Veloctiy= ln(massstart/massend)*Velocityexhaust
Where massstart is equal to the mass of your fully fueled rocket and massend is equal to your rocket completely dry or having used all fuel.
Velocityexhaust is equal to Isp*9.81 m/s ISP can be found under the info tab on all rocket engines.
For example, my ship weighs 20 tonnes, and dry my ship weighs 6 tonnes and I'm using the LV-T30 "Reliant" engine which has an isp of 310 in vacuum. Since this will be a single stage to orbit we will assume average isp of 300( (265 isp @ 1atm and 310 @ vacuum most of the flight will occur at near vacuum pressures)
ln(20/6)*300*9.81=3543.29 m/s. This rocket will make it to space (read orbit) assuming good piloting.
This is an example of how you can use the rocket equation to get to space. just remember to keep your TWR @ 1.2 minimum on the launch pad and remember TWR= thrust/mass, this number will be in m/s so around 11.772 m/s is the number you want to see. 11.772/9.81=1.2 GEEEEEEEEEES baby!
Hope this helps, sorry its so mathy....