There is nothing that diabetics have to do differently then non-diabetics to lose weight - eat well, eat less, count calories and exercise more. Its just like everyone else
Most people will gradually (or suddenly) loose weight leading up to diagnosis. Lots of time (myself included) they find they can eat anything they want and still end up loosing weight - this is due to to you body basically eating itself and not absorbing anything from your food. This pattern of weight loss is misleading because after diagnosis when starting insulin these people just keep eating they way 'they always have been' and find that they pack on the weight. This is because now your on insulin and your body is able to store and use the calories your feeding it rather then passing it unused.
Its not insulin making you gain weight, or messing with your metabolism - its your body going back to normal, prior to diagnosis is when your body was messed up causing you to not gain weight, or even loose it.