In order to build a node from source, run it and connect it to the Cardano mainnet, you need a Linux system with at least 4GB RAM and 24GB harddrive space. The RAM is mostly needed for building the node; for running it, 1GB would be sufficient. The harddrive space is necessary if you want to connect to and download the Cardano blockchain on mainnet. If you just want to set up a local testnet or connect to an external testnet, less space is needed.
If you do not have access to a computer running Linux, you can use Amazon Web Services (AWS) to create a cloud-based virtual machine running Linux for free.