There are generally one or two optimal rockets per mission. I recommend minimizing the cost of a rocket. Pick the cheapest Upper Stage that can handle a mission’s payload. Then pick the cheapest Booster that can support the Upper Stage.
Never use Supplementary boosters. Research them because they are cheap to research, but don’t use them. They’re too expensive for how little they help. You’re better off using a more expensive booster.
Don’t auto-resolve missions unless you are playing on Easy mode or are far ahead in the game. Manually resolving missions greatly increases your rewards and chance of success.
Always use Prototype payload variants when possible, especially if you are manually resolving. This saves you a ton of money.
Do every Lucrative request mission, even if you are rich. You need over 25,000c for the Mars Missions.
Perform Earth missions There are a lot of optimal launch windows for Earth missions
The game starts in January 1957 and usually ends after 2010 or later depending on the game difficulty.
The first mission is called Test Launch. This mission requires you to launch a ‘sounding rocket’ into sub-orbit. In order to complete this mission you need to research the Small Launchpad and build both the launch pad and rocket. Once this is completed you can schedule a launch date.
All other missions are similar to this one. The make progress in the game by completing missions. Complete the research required for a mission, build the rocket, schedule a launch, and then launch the rocket.
You have 3 different resources that you need to manage: Funds, Science, and Support. Support determines how much Funds you get every month. Science is used to research missions, buildings, and rockets. Funds are used to build rockets and maintain buildings. Most resources come from completing missions, but you also gain some passive Science from buildings.
Your goal is to be the first to complete the Crewed Mars Landing mission. This is the last mission available in the Missions research tree. You complete this process by completing all of the prerequisite research and missions. There are 4 other agencies that you are competing against. You aren’t required to complete every mission. If you are playing on higher difficulties then skipping some missions is advisable.