Go to a big dealer with loads of models, layouts etc. You don't need to buy from them but you do need to decide what sort of layout will suit your lifestyle and seeing loads of vans in one place will help you make your mind up. For example I love a rear lounge because I like to sit in the evening with a DVD and glass of wine, but passenger seatbelts and a dining area don't interest me, and I feel a fixed bed compromises space too much - but that's just me, you need to think about what works for you. Buy magazines - MMM is good - to get an idea of issues, tips, budgets, suppliers etc.
Don't rush into it, it's too expensive an outlay to end up with something you don't use because it doesn't work for you.
Finally, remember to become a member here - well worth it!