The first thing to do is take a careful look at the site where you are proposing to build your conservatory, and mark out the area that you think it will cover. You may well decide that the loss of garden space is more than you expected, which could prompt you to perhaps scale down your ambitions somewhat! A south facing wall is best if you wish to take maximum advantage of sunlight, and you should pick a spot that is not overshadowed by trees, since rotting leaves are not only a nuisance to clean away from your roof and gutters but they can also stain the structure, whether it is made of timber, UPVC or aluminium.
You should then decide on the design of the conservatory. Whilst this is very much a personal choice a simple white UPVC one can blend in beautifully with a modern house but look completely out of place on an older one with more decorative architectural features. If in doubt try to create a mockup on your computer so you can get a better idea of just how the addition will look. The number of available designs are almost infinite and a careful choice now can save a lot of regrets in the future.
Be very careful about selecting the builders whom you will employ. Ask them for a list of recent customers and ask these clients if they would mind letting you inspect the work, and let you know what they felt about those builders. Sometimes the most heavily advertised companies employ a lot of staff on a temporary basis which can mean that their work is not as good as it could be, so don't be put off by the fact that a company is only small provided that it is well-established with a good reputation.
Contact your local authority to ask them what planning and building regulations approvals are necessary. Get quotations in writing and make sure that they are comprehensive. Be very careful if you are offered a hand written quote with very little detail; a competent company should be able to give you a fully detailed specifications without being prompted to do so.
Finally make sure that you inform your insurance company or any other entity which may have an interest in the work. If your property is leasehold or subject to a chief rent you may need to get permission from those to whom you pay the rent before starting work.