dont ever use return to launch pad or anything always use quick save and load etc .. quick save before launch etc...
stay away from missions that are high altitudes/ and low speed etc ... for now. stick with ones that are ALT or SPEED only .. do both only if you are sure you can do it.
use F12 to keep an eye on physics
to EVA in space you need upgrade astronaut complex first ...
hit the rest button in the build menu to reset all the stage to .. logical order
missing SAS buttons ? upgrade at tracking and mission control then level up your Pilot to 3 or use tech tree to unlock mod with SAS https://forum.kerbalspaceprogram.com/index.php?/topic/186853-most-efficient-way-to-level-up-pilots/
try to just focuses on missions you can complete easily
when you finish a mission keep an eye on the recovery mod or % at the end it will tell you in the report how much money you lost ....
A good rule of thumb is to simply keep the ship between 200-300 m/s until you are above 12k meters so you aren't burning extra fuel to get through the lower atmosphere where you're going to experience the most drag. https://m.youtube.com/playlist?list=PL-7ICWbZUDjlBrsHIS8mEEj3xIMUQwlp5
different rockets work better in vacuum of Space
for large payloads: Fuel Flow & Asparagus Staging | KSP Beginner's Guide