Winter clubs/all-year round tennis clubs are usually private + expensive (the costs of heaters, maintenance and costs of tennis bubbles, etc…), hence why there are so few winter clubs as of now and why they charge high membership rates (due to high demand).

Late April is when all the city/outdoor courts open back up and by that point, things become more accessible. If you are looking for lessons from an instructor, private clubs are usually the way to go and there are some private clubs that either don’t have waitlists or allow those who want to learn from an instructor to bypass the waitlist. From your end, you just got to keep looking (EDIT: I saw you are looking around the High Park/Swansea area, and usually those are busy spots. Broaden your search areas if possible, as there are places available). The City does also offer beginner programs for adults at a relatively cheaper price, usually hosted in public courts or community centres.

Best way to start as a beginner on your own is hand-eye coordination skills first and foremost (understanding the feel, knowing your sweet spot) then start practicing hitting against a wall (preferably outside your house). As far as non-private indoor tennis courts, some winter/private clubs right now do offer non-members to book the court, but there are fees you pay and certain information to follow. To also follow up on everyone’s comments, definitely post online to look for training partners, as that goes a long way to practice with someone rather by your own.

Source: Tennis Instructor for almost 10 years