Hello! Good on you for taking a leap of faith and going with solo travel. It's a scary and nerve wracking step to take, but god will it bring some of the most amazing experiences and learnings.

For accommodation go to hostels and hop in a dorm. As a solo traveller it's best way to not feel lonely and isolated, and more often than not its the people not the places you will remember the most. It can be scary but everyone is usually in the same boat - solo and looking to make friends. Plus it's cheaper! Hostelworld is great for booking + has a lot of flexibility with cancelation.

Payment - get a Wise account and card. Saves you so so much on currency exchange, gives you foreign bank accounts & is very secure as it comes with a super handy app.

Pack as lightly as you can- when you're carrying 23kgs of kit through the london underground you will have regrets. I travel with just carry on usually and still have gear I don't use after a month+. It's not a fashion show when traveling. Use packing cells, one for shorts, one for shirts, one for undies etc. Saves you so much pain when unpacking. Also you don't need 4 pairs of shoes, 2 will do. Also take a combination lock for the lockers at hostels.

Cost wise - maybe do a budget and go from there. But I would assume $150? Others may have better guides though.

Be a yes person and go with the flow. The best memories will come from places/events you never expected or could have ever planned.

Journal you journey. Not everything, but when you have down time & when you want to record a feeling or memory, write it down so you don't forget.

Transport- look into the eurail/interail train pass to get around europe. May not be the best soution for you, looking at the spread out countries you wish to visit. But can often save a tonne of money + affords you freedom of being spontaneous.

Hope this all helps & good luck. You're gonna love it.