To get from Grand Central Station on E 42nd Street to The High Line Park in Chelsea you need to take the following route:
- From 42nd St - Grand Central, take the "Brooklyn Bridge - City Hall Express" bound train, and get off at 23rd St Station.
- Walk to E 23rd St & Park Ave S, take the "Chelsea Piers 12 Av Crosstown" bound bus, and get off at W 23rd St & 10th Ave.
- Walk North-West for 2 minutes and you will arrive at your destination, High Line.
The total fare will be $2.50, and it should take you about 35 minutes, although you should allow a little extra time for the bus journey in rush hour.
If you have a smartphone it would be well worth downloading the MTA app, which gives you real time NY travel information and helps you to plan your journey.
Another app that you might want to check out when you're getting around NYC is Citymapper.