I am a first year zoology student at Newcastle university, and I'm trying to work out just what I want to do with my degree. (Mid-degree crisis if you will)
I was wondering if I could get some advice on a few topics.

Firstly, as this is the most immediate of the decisions I have to make, what do you recommend for choosing modules. Is it better to choose varied modules, which I haven't studied before, or choose modules related to topics I have already studied, so I learn a few things in greater detail rather than a lot of things in less detail.

Secondly, I am not entirely sure what I would like to do with my degree, although I have always thought of doing further research based qualifications, (MRes or PHD).
I would like to work in areas related to conservation, biodiversity, evolution, wildlife rehabilitation, animal behavior - do you have any advice for places I could look to give me a clearer idea of the types of careers I could have in this, and the best things to be doing now in order to be able to achieve said careers.

What would you recommend for work experience that would help me in the areas I am interested in?

I am already volunteering at an organisation called ERIC North East (http://www.ericnortheast.org.uk/home.html) , which works with environmental/conservation data in order to create a "big picture" of wildlife in the north east, and I am going to try and get work experience with Kirkley Hall Zoological Gardens (in Northumberland) in the summer.
Can you recommend anywhere else I can look to get experience which will help me?

I really appreciate your time and your help,

Thank you very much,

To your first point, unless there's a glaring reason not to, the best bet is probably to pick whatever interests you most, based on course descriptions and your previous year's experience.  Don't worry about specializing or diversifying at this stage - it'll all count towards a Zoology degree.

To you second point: The best bet will be to speak to the lecturers of the coures/areas you enjoy most, and ask if they have need of any lab volunteers, or research assistance.  You may be able to get vital experience in your own University, and doing that you'll be able to try several things, and also build up good relationships with potential referees.

Your current volunteer work is definately a goot step in the right direction.

As an example, in the first two years of my undergraduate degree (Plymouth, Ocean Science), I took quite varied modules, and then ficused in a little in the 3rd year. This helped me (1) get a baseline, broad understanding of the subject, (2) let me work out what I enjoyed (or not!) and (3) enabled me to get to know more lecturers etc within the univeristy. Also - check 3rd year modules as for some you will need to have taken a particular 2nd year module.

Which modules you take in the end are up to you, but if you are thinking about postgraduate degrees, then why not have a look at what modules / research they offer. What options do you have now that might be of use? Will you end up repeating material in a second degree? 

As for volunteering, you seem to be doing well so far! I would also agree with Peter - getting experience in a lab will be of great benefit for a postgraduate degree.