As has been mentioned above, to begin with simply ride your bike.
Before you begin any training plans, get to know your bike, practise handling techniques over a variety of surfaces and terrains (depending on the suitability of your bike for terrain types) and in a variety of situations. Ride until the bike and you are an extension of one another (sounds trite, even cliche, but still true).
Training plans both outdoor and indoor are great once you’ve improved your bike handling techniques. A training plan before learning and improving your bike handling skills is a little like learning to run before you can walk. My own opinion is that training plans are to improve on what you have already learnt. You need to learn basic bike skills first, even if you are coming back to cycling as you will need to fine tune and re-practise what you once knew. Correct technique for shifting, climbing, descending, cone of movement, balancing, etc. is more of a priority.
Have you tried using your bike for commuting? Whether to work or the shops, wherever. Regular and frequent shorter rides will improve your fitness much quicker than less frequent longer rides. Build up to the longer rides later. You don’t have to commute every day at first (as you’ll be absolutely knackered by half way through the week!), just one/two days a week at first then increasing frequency until you can commute every day. Take your time, no rush.
Most of all, have fun. If what you are doing isn’t fun then you are less likely to continue. Do what works for you.