Now is the time of year to look as the prospectus' of your local colleges, as most offer part time GCSEs in a wide variety of subjects. You'll need maths, English and a science subject. Depending on your circumstances (benefits etc) you may be able to get reduced rates.
The degree course (or degree course + PGCE will take a total of between 3-4 years, depending on which route you take. I believe there are also routes that enable you work and study at the same time, but I don't know about duration).
To get into university, you'd usually need A levels, but many universities now offer one year access courses aimed at getting students onto their own degree courses. These are equivalent to A levels.
So - 1-2 years getting GCSEs, 1 year's access course, 4 years at uni = 6-7 years via the traditional, full time route.
Alternatively you could take an Open University degree, which could take from 3 years to however long you like, and around 18 months to do their PGCE. And you'd still have the GCSEs to pull in - it would probably still work out about the same, but you'd be studying in your own time and at your own pace.