Take a deep breath and relax. I have walked from Lisboa to Santiago 3 years ago on the Central Route and will walk again this late September/October. Where you start will depend on how much time you have. If you have a month you can start in Lisboa. Not that many people start in Lisboa. The first couple of days aren't the best but it gets prettier as you go. If you are a religious person, or even if you are not, you may want to take the variant to Fatima. As you get closer to Porto and especially after Porto there will be alot more pilgrims than you met starting in Lisboa. Both cities are lovely as are some of the other towns and small cities as you approach Porto, especially Coimbra.
From Porto you have the choice of walking on the Coast that has two routes that merge at times but are very close to each other, or stay on the Central route and later you can take another variant on the Spiritual Variant. It sounds complicated but it really isn't at all.
Every person who takes their first step on their first Camino has stress and worries. Do not worry because before you know it the worries will be completely gone. Pick a place to start and take your first step.
Just remember you only have the step you are talking at that moment. Don't worry about tomorrow it hasn't happened and forget about yesterday because it is over.
You do not need boots. Really good trainers will be great. Do a search for what people think about shoes and socks. They are very important. Look up
@davebugg he is the guru of feet and equipment and is really helpful and generous with his advice. Pack light and pack only what you know what you will use. You are walking one of the easier caminos and at your age you should have nothing to worry about especially if you do some training.Get a rucksack, backpack that feels right and train with your pack and your trainers.
You will meet people and some wonderful people. If you want to talk to someone open up. If you want to walk in silence, walk that way. Stay in the albergues and you will meet lots of great pilgrims. Even if you are alone and you see a few pilgrims going to dinner. Just ask if you can come. No one will ever say no. Get the
Brierley Guide book to the Camino Portuguese. I think that will help you and you can order it here, just click on Camino 2020 guides. I think he is putting out a brand new guide for 2020. On your phone you can download the Buen Camino and/or Wisely app for the Portuguese. They will have more places to stay and GPS so even if you get lost (This camino is well marked) you can find your way back. Check out Gronze.com and open it in Google Chrome and it will automatically translate it to Ukrainian, or any language you choose. It has very clear distances and reviews of alot of albergues.
Finally if you have problems or lost you are in the best country in the world to get help. The Portuguese people are the nicest, friendliest and most generous people on earth. The food in Portugal is also pretty cheap and really, really good.
Take a breath and just walk Pilgrim!