It's hard to make specific suggestions without knowing your starting point, ie is there scope for you to save £100 pm.
The Moneysaving Expert money makeover will systematically guide you through all aspects of income and outgoings so you can see where you can make savings and a few little things can add up to a significant amount - don't think 'oh it's only a fiver saved, it's not worth bothering' if you find 20 ways to save a fiver a month (or ten ways to save a tenner) etc, then there's your £100 right there.
https://www.moneysavingexpert.com/family/money-help/
There's three ways to look at it
increase income/money coming in. Can you work more, are you entitled to any benefits? Plus little things like if you switch your bank account, you can get money for doing that. It can be worth keeping a second account with a couple of direct debits going out of it that you can switch regularly to get the offers.
Then can you get any of your regular spending/bills etc cheaper. Eg mobile phones, broadband, pay TV etc. Food and drink out of the house is a potential big saving. You've already identified the drinks swap from one at a time from a convenience shop to taking water with you. Also if you get takeaways, can you swap to supermarket meal deals instead - that can be a big saver.
Then what can you cut out - is there anything you buy or do that you don't really want/need that you could just cancel?
On the savings front, if you put it away when you get paid then try to manage with what you have left. If you don't touch the £100 you've put away, you've achieved your goal.