@sunshineandstarsgirl I know you're walking but just to build on reassurance about the Tube, because it's not nice to feel like you're the only one who struggles with it...
If you're ok with busy train stations you should be OK with the tube, its just a series of train stations underground. From the overground station (eg euston) you take the escalator/steps/lift down and look for the tube maps on the wall which will show you which line and direction you want. Then follow the signs accordingly (eg northern line north / south).
The signs directing you to specific platforms will be more detailed, showing the stops along the line for each platform.
There will be barriers at which you need to put through your ticket or scan the ticket qr code on your phone, but these are always manned so if you get overwhelmed or unsure you can ask for help.
People move quickly on the tube but there are always people willing to help if you run into problems. Avoid rush hour.
On escalators stand on the right hand side, the left is left free for people to walk down without being obstructed. When you need to pause to check direction or ready your ticket step out of the main flow of people and to the side, because nothing is more frustrating that people who stop at the top of escalators or in front of ticket barriers!
The underground is warm, there will be rushes of warm air near platforms, and smell kind of dusty - I quite like it.
Even experienced tube users don't always know which direction or exit they need, so don't stress. Just step to the side and work it out or decide its pot luck. At the end of the day, the worst that's going to happen is you end up going a stop or two in the wrong direction or coming out the wrong side of the station above ground.
There are videos on YouTube for first time & nervous tube-users - like this one: https://youtube.com/shorts/y1vI4-EB53k?feature=shared
And City Mapper, as others have recommended, is a great app. It'll give you all the ways to get where you're going, by foot, taxi, boat, car, tube, overground, with directions and timings. It also includes accessibility info if you need step free access etc.
Hope this helps for the future :)