I mean, if you are really trying to understand String Theory, then you're going to have to become fluent in Classical Mechanics, Quantum Mechanics, Quantum Field Theory, and General Relativity first... otherwise the papers are going to be unmotivated and you won't understand the linguistics and you won't know how the results connect to the universe (i.e. they're more than just a sequence of symbols which we call math).
That being said, assuming CM/QM/QFT/GR are under the belt, the best place to start is Green-Witten-Schwarz's (GWS) Superstring Theory, followed by skimming Polchinski's String Theory. This is supported by my string theory professor when I took it a while ago, Petr Horava (discoverer of D-branes). From here you can supplement other notes and papers.
In Vol.1 of GWS, chapter 2/3 will explain the bosonic string theory (i.e. ignoring fermions) and BRST quantization, which leads to a critical dimension $D=26$. Then chapter 4 will fix this with supersymmetry (i.e. putting back in the fermions), leading to the actual critical dimension $D=10$. After this, gauge anomalies and compactification and dualities and D-branes can start being assessed.