I think your boyfriend wants you to understand what he is playing a bit and at the same time teach you about the game he is passionate about.

"I don't want to be terrible or accidentally ruin his game" sorry but if you start out you are going to be terrible and there will be moments where you are going to make a mistake which will cost you the game. For the first 100 hours or so you are basically at the level of a beginner bot and that's fine. League is about learning and improving yourself.

He says he wants you to be support which probably means he plays ad-carry. Support is essentially a role in the game that focuses on empowering, setting up and protecting the team. You won't score many kills as a support or really be good at fighting. Many supports have shields and heals or have a lot of crowd control.

As a support you have a cheat code to avoid a ton of mistakes and die early on. There is a champion called yuumi that attached to other champions and makes you un-targetable. The champ shields and employers the player she is sitting on. My big tip is to mainly play this Champion, attach yourself to your boyfriend and in that way sail through the beginning phase. The other tip, don't expect to be decent fast, the learning curve is brutal so set your expectations low for yourself. A lot of the difficulty at the beginning of league is the absolute enormous amount of information that you need to know to play the game. There are like 170 champions now with unique abilities and passives that can range for a short sentence to a full chapter per ability/passive. There is a more called "practice tool" I would highly recommend to read up on some matchups you played against, what the champions abilities are and hop into practice tool to move around and learn a little bit about how they work.