Next would be size, how big to fit the space, and the cost must be considered, first priority is cost, how big can I afford, second priority can I fit it in the space I have
OK with that decided, and I would recommend buying the biggest you can afford that will fit the space you have.
the next thing is to order one, there are lots of ways to do this, garden centres sell them, or you can buy them on the web.
A google search will find thousands of places willing to sell you one
OK so you already decided on what you can afford ............. don't forget to add the cost of foundations, never included in the cost of buying, does your property slope? what type of floor do you want? do you want to insulate the base? all these factors can add a huge amount to your budget
OK so you factor that in to the cost, time to order and build..............
foundations will need to be levelled and square or things will not fit
you might need to brick up to get the foundations level
next stage would be to add insulation if you want to
after that you need to add a floor, in this case concrete
the big day arrives and your greenhouse is delivered
first read the instructions twice if you need to get some help 
lay out all the bits and prepare to assemble
follow the instructions
assemble as required
another cost to factor in is where to put the plants ...............
so now you have a shiny new greenhouse, where do the plants go
right back to stage one, did you add the cost of staging and shelving ? ......................no? ok can you build your own? if not you need to factor this in too
time to add the glass, choices when you bought .............. horticultural glass, comes in small sections with clips in-between............cheaper than toughened glass which can be bought full length.......... metal clips or bar capping
bar capping holds the glass more securely but costs more
the finished item can cost a lot more than you budgeted for
but can add a big smile factor
if you have to worry about the money you can't afford it
.........if you love the plants you can 
