My gran wants to put some money in bitcoin for me

So, my grans mate set up an account for her to invest in bitcoin and she’s made a bit of money. She said she’d like me to make an account and she’ll give me money to put into it. I feel like this is a good idea to help me save up for my future (I already have a savings account as a cash ISA).

Any recommendations for good apps? I’m looking to just put some money in and let it sit and grow over time.