Does your employer have a salary policy? For example, a previous employer aimed to pay local average + 10% for clerical roles and national average + 10% for professional roles. Current one is between 70% and 130% of national average (which gives them a lot of leeway.)
Do you know what the going rate for your role is on average? What factors affect it? For example, my role usually offers between about £30k and £75K, depending on the company, location, etc. To get the top rates, I'd have to travel into London (and plenty of people do commute from here,) but I don't want to do that. I am paid acceptably for what I do, and despite the moaning from my colleagues, it's several thousand more than a lot of other jobs doing the same thing in the area - and I do know, as I was applying for lots of them 18 months ago, after redundancy.
Armed with those two pieces of knowledge will let you know where you should be in the market, and where your employer is willing to pay on that scale.
Are you direct comparators the same sex as you? There may or may not be an element of sex discrimination at play. Are there any other distinguishing factors, like different qualifications, more experience, etc?
You can't use confidential data accessed for your role to argue you should be paid more. You can use things like you've taken on more responsibilities or market averages. It's difficult to argue you have more experience when you're only new to a role, and you may need to wait. I've worked for employers that won't give payrises in the first year of employment, and it's very usual for them only to do pay rises at a set point in the year, so you need to be in line with that, remembering that negotiations and discussions around it will start months ahead.