The first thing is to sit down and write down all your expenses. Review your last few months bank/credit card statements to see where your money has gone. Group them together in categories so for example I have groceries, work snacks/food (for what I buy at lunch/breakfast at work), travel, clothing, toiletries, fun (can be books, cinema, etc). When you see that your money goes to those different places, you can start looking at where you can make small changes. I put a set amount each week into a separate bank account for me to spend and use whatever I need it for.
For example I saw most of my money was being spent at work on breakfasts. God I love a Pret croissant!! So I found easy fun recipes I could bulk cook and freeze for the mornings.
I will also have a look at annual expenses such as insurance, MOT, christmas, birthdays etc. Think ahead about what you will have to spend over the pay period.
Some people swear by the 50/30/20 budget. This is where you spend 50% of your after-tax pay on needs, 30% on wants, and 20% on savings or paying off debt.
There is lots of good advice on Money Saving Expert forum. I also found a lot through youtube videos. Mamafurfur is a popular account. There are lots of money bloggers/debt free journey community on instagram too. I enjoyed following people with a similar state of mind which helps me focus putting lots more to savings.