If you can afford to do so from a tax perspective, it will be best to make Roth 401k contributions. Pretax 401k contributions reduce your taxable income in the year of the contribution whereas Roth 401k contributions are made with after tax funds, but the earnings grow tax free.
See following website to learn about the Roth 401k rules.
as a 24-year old you have 36+ years to grow your wealth tax-free. While you are giving up some tax benefits now, we don't know the tax rates 30+ years from now and it unlikely that taxes are going down... so you can't go wrong contributing at least some of your earnings into Roth.