Day tour of Tallin

Step one: Learn how to spell the city you plan to visit. The name of the Estonian capital is Tallinn, with two "N" in the end. People can get offended if you only use one N.

Step two: Just buy a ferry ticket. There are several companies operating the route and departures are frequent. If you want a tour in Tallinn, here are a few suggestions: https://visittallinn.ee/eng/visitor/see-do/things-to-do/tours

Agree with Badger that you shouldn’t need a private tour. Just book ferry tickets and if you want a walking tour, choose one after you know the limitations of your ferry times. Remember that it will get dark early in March. I have 7 days in Tallinn this month (solo also) and am excited!

Sometimes I just want someone else to take care of the details. I found this option for $142.56:
https://www.getyourguide.com/helsinki-l13/helsinki-tallinn-guided-day-tour-with-ferry-crossing-t432177

For the price, they are providing the round trip ferry ticket and a guide who will meet you at the ferry port in Tallinn. You can decide whether the convenience is worth the cost, in comparison to doing it yourself.

So true - sometimes you just want someone else to do the logistics!

But if you DO decide to do your own thing, here are some other tour possibilities once you are in Tallinn.
https://www.freetour.com/tallinn
And here a link for ferry times and prices.
https://www.directferries.com/helsinki_tallinn_ferry.htm?&utm_source=google&utm_medium=cpcdfgeoo=true&gad_source=1&gclid=EAIaIQobChMI29qgr9eliAMVuizUAR0a1g-9EAAYASAAEgISWPD_BwE&gclsrc=aw.ds

Yes, sometimes you want to go for the easy option. But you will have many more options for tours if you look for tours in Tallinn. If you want something starting in Finland there will be a lot less to choose from. And buying a ferry ticket is not complicated.

I took 2 day trips with a wonderful locally owned company called EstAdventures https://estadventures.ee
One took me to manor houses, a ruined monastery, the town of Haapsalu and an abandoned Soviet military site, another to the bogs, Jaagala waterfall and a fishing village along the shores of the Baltic--plenty of time for a bit of beachcombing for sea glass, and lunch stops at excellent restaurants. They are small groups--I went in mid September, so tourist season had wound down significantly, and one trip was me and a couple, the second was just me and the delightful owner--by then we were old friends. They do trips around Tallinn sites as well, and the price was affordable--both trips were very enjoyable/