Buy Weapons and Mechs in your current tier. For example, if your in tier 5 and you bought an epic weapon or mech in that tier, it will give you 4 stars. You can also rank up your mech or weapon in the tier your in, but it will only give 1 star per rank

If you buy a weapon or mech from a previous tier, it won't give you a star. Buying a pilot won't give you a star.