At the core of it - you're going to be leading engineers. A lot of how this leading happens depending on the organizational makeup of the firm you're going to. (This is super super important!!!).

So figure out the organizational culture before you join. Understanding this helps you understand the confines within which you will operate (for the first couple of months, atleast). If you're replacing someone, understand how they worked, what gaps were there in that person's performance and how you could cover those. If you're the first tech lead, then figure out what problems they're having that necessitated the creation of the tech lead position.