Well, you need Kerbal Engineer, its a must, because it tells you the thrust to weight ratio and the Delta-V when you are putting your rockets together. Its a very useful tool to have. Also, are you using B9 and Ferram Aerospace as well? I ask because Ferram can make things more difficult for you because of realistic flight physics and the whole aerodynamic failures option (you can turn that off btw so you do not have to worry about your rockets exploding out of nowhere because the stress became to great) and as for B9, that pack adds a ton of parts that you need to get familiar with at first. I recommend to start slowly, build some easy rockets with stock parts to get the science for better tech, keep in mind that Kerbin also has biomes that you can explore for science. Dont rush things, it will be a long way to get all the tech in the Interstellar tech tree anyway.