Do not forget to bring your swimsuit and opt for real good shoes and not flip flops to avoid to slip because the more you climbs, the more the path can be rough with roots and stones and is slippery. Try to come in the morning, ideally before 10 am because after, there are a lot more people, Erawan Waterfalls are the most famous impressive waterfalls near Bangkok. Some people go back and forth in the same day from the capital just to visit them even if I do not recommend it because it would be a shame to not take more time to discover the Kanchanaburi province. Note that depending on the time of year, the waterfalls are more or less impressive, the least interesting months to go are at the end of the dry season and the beginning of the rainy season (between February and early July in general).
See here a list of accommodations close to Erawan National Park
To go visit Erawan National Park, you can also book one of the tours above.