Mark out in your garden where the conservatory will go to get a feel for how much space it will take. Always calculate your internal floor area. Most suppliers will quote sizes based on external sizes. A few pegs and string will do the job.
Browse the internet and send off for some brochures to get an appreciation of different designs, before you buy a conservatory.
If you want it to last and keep looking good, buy the bestone in the most expensive material your budget allows. The cheapest supplier rarely supplies the best product or the best service. Allow 10% for 'extras' or additions you may make later to the order.
Get at least three companies to quote you for before you buy a conservatory.
Ask to see some of their previous work. Speak to their customers to get feedback on how well the installation went, before making the final decision on the supplier.
Get the ground and building work right. Get this wrong and you have inherent problems that will never go away.
House Insurance. Remember to inform your insurers of your new conservatory / sunroom addition. Cover for complete re-building should be arranged.
Creating the right look. A conservatory should look as if it's always been there, rather than bolted on. Choose the conservatory style that complements your home.