Avalanche is an incredibly lightweight protocol, so the minimum computer requirements are quite modest.
Hardware: 8 core CPU >= 2 GHz, RAM: 16 GB, Storage: 200 GB free space.
OS: Ubuntu >= 18.04 or Mac OS X >= Catalina. (Ubuntu versions older than 18.04 may work but have not been tested.)
Network: IPv4 or IPv6 network connection, with an open public port, capable of at least 30Mbps throughput.
Requirements scale with the amount of AVAX staked on the node. So if you have a big node (100k+ AVAX staked) get the biggest instance you reasonably can to ensure your node doesn't fall behind and put your staking reward at risk. If you have a small node (less than 10k AVAX staked) you can run a healthy node on below-spec computer.