Starting my own business

I'm interested in designing greetings cards / making gifts. I wouldn't know where to start when it comes to starting a business. Are there any rules eg. can I use popular tv characters on them , can I buy products from shops and put it in a gift set such as Cadburys chocolate.
How would I budget things and what's a good starting point ? should I make all of the products such as 10 of 5 designs or make 1 version and wait for orders to come in ?
I think I would begin by selling on Etsy but I would also love to run a market stall and sell at events especially Christmas markets. Do you need permission to sell things at these events ?

I'm interested in designing greetings cards / making gifts.
Greeting cards and making gifts is a very niched market, so you're going to have to really stretch yourself. There is also a lot of competition, so you would need to bear that in mind.

I wouldn't know where to start when it comes to starting a business.
I think something like the FSB would be a good place to get general business and legal advice from (you pay something like £150 a year for them, but you can ask them as many questions as you want and for as long as you want). See: https://www.fsb.org.uk/join-us/membership/fsb-member-benefits.html
If not, I would speak to someone from something like a business hub in your local area or a place where they give free advice to people looking to start their own business. These are government sponsored organisations, and they tend to be very good.

If you don't have any business knowledge to begin with, it's a good idea to read up on the basics so you're acquainted with it first. I would recommend picking up a textbook on A Level Business Studies or CIMA's BA Cert. You don't have to do the full qualification, but the material included in these courses would give you a good foundation for general business knowledge.

For something specific to your sector/market, I would try to get a business mentor or someone who has succeeded legally in the market/sector to learn from. General business advice and knowledge can only get you so far.

Are there any rules eg. can I use popular tv characters on them , can I buy products from shops and put it in a gift set such as Cadburys chocolate.
I am not a qualified solicitor, so I wouldn't give any legal advice. However, these would likely have copyright issues, so check with a solicitor about it.
If you want to include their material onto your products, then you woud likely need their written permission prior to doing so.

How would I budget things and what's a good starting point ?
The following are good articles on budgeting:
https://www.zoho.com/books/guides/how-to-create-a-realistic-business-budget.html
https://www.nerdwallet.com/uk/business-bank-accounts/how-to-create-a-budget-for-your-small-business/
https://www.tide.co/blog/business-tips/business-budget/
https://www.fsb.org.uk/resources-page/how-to-create-a-small-business-budget.html
https://www.wordstream.com/blog/ws/2022/09/28/small-business-budget

Personally, I would use a cash flow forecast as it shows a breakdown of the timing of the cash flows as well as the budget. See:
https://www.british-business-bank.co.uk/finance-hub/how-to-create-a-cash-flow-forecast-in-4-steps/
https://www.cashanalytics.com/cash-flow-forecasting/

should I make all of the products such as 10 of 5 designs or make 1 version and wait for orders to come in ?
The rule of thumb is to create a prototype of the product and see if there is a market for it. If there is, then do a selection of designs, see which ones sell, then market those that do well and cut those that don't (e.g. 10 designs then cut it down to say 3). I don't know what the card or gift market is like, so I would rather if you learn this from those who have succeeded in the market.

Do you need permission to sell things at these events ?
Generally, yes. You can't just set up stalls and markets on a whim. I would find out who the organiser for the event is (generally it's the local council, but do check) and see if you need to rent a stall or space in the market. Again, gifts and cards is a tough market to crack, so be aware of this going in.