I think you are making the right decision for now.
As others have pointed out, taking on an employer requires you in a whole raft of administrative tasks which will consume your time. In particular is the process by which you have to deal with supplying information and making regular payments to HMRC (for tax and NI for your employee).
Taking on an employee is a big step and in the first instance it is a low risk solution to use someone on a self-employed basis. Ideally that person should have another job working for someone else either as an employee or a self-employed role.
The simple answer to how taking on an employee affects your own tax position (for info for later), is that you pay income tax yourself to HMRC, and the amount you pay is based on your net profit. That net profit is the difference between your businesses income from all customers, less all your allowable expenses and costs. One of those costs is the whole of your employee costs, including pay, NI, sick pay and holiday pay.