We adopted our beautiful boy earlier this year.
We went to information evenings at the Local Authority, then another one a week later with Barnardos. We went with Barnardos, because we just loved the warmth and enthusiasm of the team. We continue to be impressed - every person we've dealt with has been great. When at the info evenings, grill them about what post-adoption support you will be able to access.
We were advised to keep our 'criteria' (number and ages of children) as wide as possible initially, as once we were going through the process, it would become clearer to us what would work best. This was good advice, and they were right on this.
We have birth children too, and the biggest shock has been how different parenting this child is to how it was with our birth children. Almost all adopted children will have additional needs, and it's important to be open and honest with each other about what you are up for taking on. Our son has some issues that hadn't been picked up on until he came to us, so be prepared for the unknown too.
Read everything you can get your hands on. Sally Donovan's books are a really good starting point. I found reading real life experiences of other adopters helped keep my expectations fairly realistic. There's a lot of Disney-esque rubbish out there about adoption, and family and friends can get very dewy-eyed and sentimental about it, not realising that it's actually pretty gritty stuff.
Last bit of advice, if you're doing this in a couple, have as many meals out and weekends away together before you adopt as you can possibly manage. Enjoy the last bit of time together as a couple while you can.