I moved from a job that did not have a 401k plan to a job that does
have a 401k plan. I have not contributed to any other IRA or 401k so
far this year.
Because you have not contributed any money to a 401K plan this year, you can put the full amount into your 401K by the end of the year. Keep in mind that that for the purposes of the $18,500 or $24,500 limit it includes all the money you contribute to a 401K, 403, 467, or US Government TSP.
Your contribution to a 401K plan doesn't impact the amount of money you can put into an IRA, but participation in a 401K plan can impact if the contribution to an IRA is deductible.
Note that some companies limit the maximum percentage of each check that you can contribute into the 401K. Therefore the limited number of checks left in the year could make it hard to reach the limits. Also a paycheck with a date in 2019 is a contribution in 2019, even if all the working hours are in 2018.