I haven't built a summer house as such but I have built many timber buildings.
First thing is the foundation/base, if its already in place is it tight up against a perimeter fence/wall if so do you have enough room to work behind it?
If so before you even start with any wood work take a good long look at the plans or instructions, I am assuming its a kit and you will have to build the frame then clad it.
Depending on how good the plans/intructions are they will probably give the lengths of various timbers I would check them, some of these kits are good and some not so good.
When you get to the building stage lay out your first few stages and get an idea of how they go together then make a start. Take your time and be methodical in your aproach.
If your not familiar with this type of work dont rush it, its easy to make mistakes (I have the T shirt)
On a job like this many hands can make light work but be aware people can interpret a set of instructions differently.