I will probably recommend csprimer, this is hands down one of the best introductions that I found and I recommended to a lot of people who reached out to me. The order I will probably recommend is the following
1. Programming: Beyond the basics
2. Computer Systems
3. Operating Systems
4. Algorithms and Data Structures
5. Computer Networks
This is going to take time but it will be worth your time.
For discrete Math I sincerely recommend you to take a look at this book "Discrete Mathematics with Applications by Susanna Epp. This is an excellent book and it has good problems.
I will also recommend you to look at https://fa24.datastructur.es/ this is a very good beginner course on data structures that is completely free by UC Berkeley
1. Programming: Beyond the basics
2. Computer Systems
3. Operating Systems
4. Algorithms and Data Structures
5. Computer Networks
This is going to take time but it will be worth your time.
For discrete Math I sincerely recommend you to take a look at this book "Discrete Mathematics with Applications by Susanna Epp. This is an excellent book and it has good problems.
I will also recommend you to look at https://fa24.datastructur.es/ this is a very good beginner course on data structures that is completely free by UC Berkeley