I have built a few outdoor buildings for other people but bought a shed for myself as it worked out cheaper than buying the materials as I wanted it to be pressure treated. I erected it myself, but it wasn't as fun as building from scratch.
If it's a big shed i'd rent out a nailgun when you're ready to clad it. It'll make the job a lot faster and they are good fun to use
. If the cladding is very dry then leave space for it to expand, If its damp then you can butt them up tight. If I was building one from scratch i'd take the opportunity to stain the cladding before fixing it to the shed so you can coat the tongues and grooves for extra protection.
Have you got a link to the plan you are going to use?
One thing I would suggest is to build it in sections, that way you can take it apart if you ever need to.