I am not sure if I should wait till I have multitudes of users, or just start monetizing it now.
My first instinct would be just to allow it to be monetized by ads. I only need it to pay for itself really, but I have already built in a backend for paid subscriptions just so I could reuse the code for a future project so that is always an option. I have created a poll in the sign up process to figure out what most users thought about which option was better.
I don't want to disappoint you but ideally you should think about the monetization BEFORE you launch the website.
In the case you start with free membership, going to paid membership for everybody will probably make leave some (if not lot) of your users.
So there remains the possibility of launching the freemium membership. But you will have to find the "+" that you will bring to the paying members in comparison to the free membership. The difficulty is that if it wasn't foreseen before you will have to find some new feature (good enough to ask money for) and if your web is already at "100% as expected", it may be difficult.
In that case, the simpliest could probably be the premium account without ads. This kind of freemium is quite usual but it requires members really willing to pay (as it is so simple to just use AdBlock or other ad blocking module in the internet browser).
Please recognize that this question has many opinions, and no experts in the sense of sources of assured success. If Facebook struggles, and Google struggles, and Microsoft struggles, then nobody can be hired who knows the answers. (I get hired sometimes by big companies with pricing problems - but the expertise I have sorting out competitive pricing issues gives me a language and toolset, but few answers, for the startup case.)
So there are plenty of opinions to study: here are some of mine.
Many entrepreneurs make the mistake of waiting to think about monetization after products are built. Relying on advertising as a revenue stream only works if your traffic is through the roof....which is likely not the case for all but a handful of startups. Test monetization as soon as you can! It's not a business if you don't make money...it's a hobby.
It will likely take a while to monetize your business, maybe more than one year after the time you think. So you should start sooner rather than later. You'll also want to get all the payment processing in place, figure out your model, tiers, etc. Some folks may suggest that you shouldn't change your pricing, but that doesn't sound smart to me, it's all part of the discovery process. Also as your project matures, you will likely see more expenses and that may provide more pressure to generate revenues.