There are a number of options:
You should be able to tap in and off using a Visa or Mastercard debit card, though bear in mind some foreign-issued debit cards won't necessarily work, despite what the official blurb says.
9292.nl or the 9292 app is invaluable for public transport planning as it covers all types of transport, with real-time updates. You can also buy e-tickets on it, though there is a small admin charge.
NS.nl or the NS (Nederlandse Spoorwegen) app covers the Dutch rail network. Again e-tickets can be bought on it, with discounts sometime available if you buy in advance.
For Amsterdam itself, it may be better value to get a GVB travel pass which covers the city itself. Similarly, an Amsterdam & Region Travel Pass can be bought for the city plus a considerable area around it, if you wanted to venture further afield.
For Keukenhof, I think there's a travel section on their website covering the optiobs.
If you want transport advice once you arrive in Amsterdam, there are tourist info offices both at Schipol and outside Amsterdam Centraal.
Have a wonderful time!