Start with accounts, because this is where the dependency is most literal. Everything should be registered to the business rather than to you personally, with an administrative account you control and individual accounts for everybody else. Shared logins are the thing to eliminate first: they prevent you knowing who did what, they make removing access mean changing a password everybody uses, and they are the reason offboarding becomes a scramble.
Then look at permissions, which barely matter with one person and matter immediately with two. Most tools distinguish between somebody who can view, somebody who can change things, and somebody who can change the account itself. Deciding those levels before a hire is straightforward. Deciding them afterward, when somebody already has full access, is a conversation nobody enjoys.
Consolidate where things live, because the practical obstacle for a new person is rarely permission and usually not knowing where anything is. Files scattered across a personal drive, a cloud account, and various email threads are reachable in principle and unusable in practice. One structure, in one place, with a naming convention, is what makes a second person productive in days.
The tools most likely to need replacing are the ones priced or designed for a single user, and it is cheaper to move before a hire than during one. Anything without a concept of multiple users, without permissions, or without an audit trail will need changing, and doing it while you are the only person affected is considerably less disruptive.
Then write down the three or four things somebody will do most often, as you actually do them. That documentation is what determines whether a new person is useful in days or weeks, and producing it is usually the moment you discover which processes only work because you make an undocumented judgement partway through.
Cost it before you hire rather than after, because per user pricing changes the arithmetic more than people expect. A stack costing a manageable amount for one person can double with a second, since most tools charge per seat rather than per business. Adding those figures to the salary and the employer contributions produces the real cost of the hire, and it is worth knowing before the offer rather than at the first invoice.