There are two ways to get access to a Central install. You can pay for official managed hosting on ODK Cloud, or if you are comfortable running Linux servers, you can self-host Central for free on your own infrastructure.
ODK Cloud (get started here) doesn't have trials, but we offer 30-minute personalized demos with someone from the core team. We also offer a 14-day, money-back guarantee. If you try it and it's not a good fit, we'll give you your money back.
For self-hosters, we recommend using DigitalOcean (install instructions here) because they provide a $200, 60-day credit and a referral program that helps support ODK.