i need to build a garden shed / workshop because i can’t really afford to buy a ready made one .what is the cheapest yet still strong and lasting way to do this . it needs to be at least 10ft x 10ft with double doors,it will be stood on a concrete base so would be no need for a floor.

Why don’t you try and buy a used one instead of all the chew of building it?

Also have a word with your local DIY store because they will damaged stock which they sell off cheap, many sheds are delivered with panel damage and are easy to repair.
I used to manage a big store and sheds are a bit of a pain, some are delivered with the wrong panels so again this might be an opportunity for you.
You can join panels from two small sheds together to get it to your size.

Another option could be to use fence panels but they will need some extra support.