Best to get it sorted sooner rather than later if you are likely to need it soon. When I contacted the church I'd been baptised in, the records had been archived, and I had to apply to the diocesan central archives to get it.
Contact the County record office for the county that you were baptised - the parish register should have been handed inrto the office when the register is full - obviusly this will depend on how busy the church has been since and how old you are
The cost of a baptism certificate is £12 - it is a standard price across the counrty.
The above is for Church of England baptisms - a not non conformist e.g Catholic Methodist etc
A marriage certificate - there are several place to apply