I'm a final year student who has secured a tax grad role for 2017, so hopefully this is helpful.
First of all, well done on thinking about this already, this attitude will definitely help you in the coming years!
Being treasurer of a society and having a part time job are fantastic things to put on your CV, especially if you know how to make the most of them. Consider the required skills you need for the roles you are interested in securing and then see how your current roles have allowed you to develop those skills. Many people show their suitability for the job by showing their ability to apply skills they've learned elsewhere to a new role/sector.
In terms of uni itself, I don't know what you're studying so I can't give you detailed advice, but focus on making sure you are on track for at least a 2:1. The phrase "first year doesn't count" is only applicable to degree classifications, but it DOES count when trying to secure internships and grad jobs. It almost counts even more than your final year, because most people are applying for grad roles before they have any final year grades, so they are being considered based off of the first 2/3 years of uni.
Look for any and all opportunities in the field you're interested in. Don't just wait for an opportunity to be advertised, go to networking evenings at your uni and chat to people at firms you might want to work for. Ask if they have any opportunities for first years, and if not, ask if they would be willing to allow you to shadow them in their job role. Send speculative applications off to smaller firms, or ask to shadow them too. Make use of any career mentors you may have at your uni.
If you don't already have LinkedIn, GET YOURSELF ON LINKEDIN. If you don't have anyone to connect with yet, a good first port of call is to begin connecting with student recruitment officers in the companies you want to work for. That way you can ask them questions and show your interest. I have a few connections in the areas you've mentioned that may be of interest, so you can PM me for details if you'd like.