It is all about first impressions. Charities want volunteers that are positive, friendly, and eager to help out in any way they can. They are highly unlikely to turn you down if you want to volunteer with them as there are a shortage of volunteers at some charities and they need all the help they can get. As long as you put in the work, show up on time, and are willing to help promote specific deals and earn money for the charity, then you will do just fine.
When it comes to working with a charity on a project you are doing, which is what I did with Tiny Tims, more planning is required. You need to think of a brief, which is simply an outline of what you plan to do with the charity, what you hope to achieve, and how you hope to achieve it. After you have all of this sorted, you need to set a date and time to meet with someone at the charity so that you can pitch your idea and see if they are on board.
If they are, then you have a brief to uphold. You need to make sure you are professional and carry out the project to the best of your ability. If you do this then I guarantee you will be helping a great cause that could make a huge impact.