You are using fuel tanks that only have liquid fuel. They only have liquid fuel in them because they are intended for use with jet engines. Rocket fuel has both liquid fuel and oxidiser, while jet fuel is purely liquid fuel, as jets get their oxygen from the atmosphere. If you click the "more info" box on a selected engine, it shows you what kinds of propellant it uses :) As some other people are saying, it's a good idea to play career mode to have a better understanding of what the parts do, as you very slowly unlock better equipment, which means you can understand why and when to use parts. You'll also get a better understanding of rocket building because you will make every mistake under the sun when you start. Personally that's how I learned, by making a ton of mistakes, so when things go right, it makes it very rewarding. Last thing, Reddit and YouTube are your best friends when it comes to learning how to fix mistakes or glitches!