The lemmy docs have a version that sets it up via CDK. I would not do that as it’s needlessly expensive for experimentation.
Cheapest AWS option is set up an EC2 instance with an elastic IP, follow the wizard AWS gives you. Ubuntu OS will be the most accessible. Set SG to allow http (port 80)and https (443) from 0.0.0.0/32 and Ssh only from your IP address. You’ll need to generate a keypair as part of the setup. and then you’ll ssh in and follow any generic lemmy get started guide (probably the easy install script).
The free tier instance type will get you started but it’ll start giving you problems after not too long. Probably need to go up to like a t3.medium to avoid it occasionally freezing up on you.