I knew very little when I started, and simply phoned the council, who talked to me and sent me information leaflets, then invited me to an open evening. Went from there and never considered anything else
Now it's really easy to do a quick internet search and pick up every agency in your area. You could go with your LA, a neighbouring LA, or a voluntary agency, depending on where you live. It might be well worth contacting lots of agencies which would give you the best idea of who to go with. Some are less efficient than others, some might only be looking for a certain kind of adoptive parent (e.g. might have lots of disabled children coming through the system so only want parents who could adopt those children) and so on
Voluntary agencies don't have children, they approve adopters then you have to find a child from an LA needing adoption. Because the LA's spend a lot of money assessing their own adopters, they almost always look to their own adopters first when a child becomes available for adoption. Then if they are a member of a consortium (a group of LA's and VA's which work together and can pool available adoptive parents) they look there. Last of all, they might look elsewhere, for instance profiling the child in an adoption magazine or online. The only exception I can think of would be a child who needs to be placed a long way from their birth family, in which case they may go straight for national magazines
What that means in that VA's usually deal with hard-to-place children, who none of the parents approved by the LA could adopt. Older children (4/5+), children with physical disabilities, children with emotional problems, children from ethnic minorities and sibling groups, especially with 3+ children
If you are white British and want to adopt one or two children aged 0-4 without any serious issues, then you probably need to be with a council, either your own or neighbouring county. Many VA's don't even assess adopters who don't want hard to place children
On the other hand, if you want a child aged 6-10, or a sibling group of 3 of any ages, or a child with moderate-severe special needs, then you may be better off with a VA - they tend to be better resourced and have better post adoption support than LA's