Ask HN: Resource to learn to manage a server?

I always used shared hosting, but I am planning to launch a project which may receive a lot of (concurrent) visits* (I already talked with clients and they want to use the product). I am more a code guy, so I bought a droplet in Digital Ocean and I managed to handle it via SSH, installed apache, mysql, postfix and that stuff, but I am not sure in the maintain part. I'd love to get some resources where I can learn, if necessary, how to config it to work properly - from basic things just like how many RAM or CPU do I need for X visits or process, to maintain it good and be able to get 1000 request per second (if I got lucky :D ) Thank you and sorry if this it not the place, but I thought a lot of developers may be in the same situation I am! PS: Of course, it would be great if my project grows and I can pay for a server guy, but not yet.