Helix Bridge is free and can be reached on foot from Bayfront station (CE1/DT16). I think it's much better to be visited after dark. If you go during the day, there are many attractions at a walking distance: Gardens by The Bay, ArtScience Museum, Singapore Flyer, and Marina Bay Sands observation deck (on one side), famous Merlion and "Esplanade - Theaters by The Bay" ("the durian building") on another. And if you're into brands shopping, check out (expensive!) Marina Bay Sands and Esplanade malls. As for planning your itinerary in Singapore, I recommend gothere.sg which is available as both web site and a smartphone app. It will show you various travel options (public transport, taxi, etc.) including times and prices. Singapore is a great place so good luck and have fun!