The general advice with any workplace pension is always sign up and pay in at least whatever it takes to get the maximum employer contribution.
In your first year of RM employment I think your contributions are fixed at 5% with 3% from RM. But you also benefit from tax relief and salary sacrifice(RM call it PSE), which means each £1 gross contribution from you only actually costs £0.68.
So for example, if you're paying in a gross of £10 per week(the amount on your payslip) that would only cost you £6.80 because your income tax gets reduced by £2 and you NIC's by £1.20. Plus you'll also get £6 from RM. Meaning you're getting £9.20 in 'free money' each week.
It might not be much, but every bit counts.
I think there are around 12 funds to choose from, but this link will tell you more: https://www.scottishwidows.co.uk/save/royalmaildcplan/