Step 1 would be to establish which BTEC you want to study. There are lots to choose from, and it can get very confusing.
Are you looking to do a single BTEC which is the equivalent of three A levels? If so, then you want a Extended Diploma. You could also opt to take two or three smaller BTECs, in different subjects. If so, you might want to look at a Certificate, Extended Certificate or Diploma. Regardless, to be at the same "level" as A levels, you'll want them to be Level 3.
Then you need to decide which subject you want to study, and then find a college local to you which teaches that BTEC.
One thing to remember it to think beyond your BTEC too. What is it you want to use the qualification for? It could be to apply for jobs, apprenticeships, or degree courses, for example. If so check they'll accept the BTEC you want to study. This is important, particular for any potential university applications, as they can sometimes be very fussy and will accept some BTECs but not others.