You have the rules ready to go.
Miniatures built and painted.
Hex maps laid out.
How do you put together that first game of Battletech?
Certainly one can just throw down some mechs and start blasting away.
Design a custom mission of unequal sides and play away.
But for most games, there is a formula used- either battle value or tonnage that allows players to select forces, build sides, and still have the game be as fair as possible- with a few considerations.
The first and most often used is battle value- each mech, vehicle, and unit in the game has a calculated value based on tonnage, armor, weapons, etc. which is called battle value.
We agree to play a set value game- say at 4000 battle value, and each player can select units and mechs added up to this limit.
The second, less used method is to add up tonnage and set a cap to it- kind of a pre-battle value but not as exact.
Another point to keep in mind is the idea of total warfare in Battletech.
I find it helpful to agree on the types of units one is able to bring ahead of time to the table- just mechs? Mechs and tanks? What about air support or artillery?
This way all sides know and can build a list based on tactics and what one feels like playing or exploring.