Hi that sounds great what you want to do, a lot of people come into fostering looking to support younger children, and there seem to be many teenagers/young people who need support.
The first thing to decide is if you want to foster for a local authority, or an independent agency IFA. Initially, local authorities look to place children with their own carers, so if you are with them you could be more likely to get placements. Some local authorities (including one I am with) really need placements for teenagers and pay experienced foster carers a wage, so it might be worth asking if this is available. You could foster for a neighboring local authority, it doesn't have to be where you live, within reason!
Some IFA's are charities, or non-profit organizations. They are paid by local authorities to provide placements, and some are businesses that make a profit. It would be up to you if you agree with the ethics of this.
IFA's don't have their own children, they are referred from authorities so you could get a 'harder to place' child. They might be older, have special needs or behavioral issues. Agency social workers could have less carers to support, so you get more support and help, and they may 'pay' more.
It would be ok to approach a few agencies, BAAF (British Association of Adoption and Fostering) have links from their web page, and ask questions about what support is available, how quick they assess, what sort of placements they offer.
Just have a chat with them, read their info packs, maybe go to open days see how you feel and who you are most comfortable with. As long as you don't start an assessment with anyone, that is fine. Hope you get on ok.