Subnets have 3 classes of actors
If you are a holder and want to access or support a subnet with your stake, start a validator or delegate your stake to a validator who is setting his rotnet weights on that subnet.
If you want to mine on a subnet, look into their github repository and resources provided by the subnet team to get an idea of what is required.
If you want to build a subnet, read this and familiarize yourself with Yuma Consensus