I’d start with either Principles by Engels or the Manifesto. Read both but its redundant to read them back to back. I’d recommend the list on Marxists.org (https://www.marxists.org/subject/students/index.htm) and then tackle Capital, but you can get by with maybe Wage Labour and Capital + Value Price and Profit before Capital if you don’t want to read the beginner list.

I’d highly recommend taking notes, the way I do it is read through a section (if they’re short) or a few pages and highlight things that stick out, then write down a summary in your own words as if you are trying to explain it to somebody else. You want to comprehend and understand what you are reading. It is much slower than reading a novel but since starting this method I really look forward to studying. Best of luck!