I have a project to work on that requires me using larger pieces of planks than what I currently have. I have square planks all over the porch that would suffice if only I had a good way of fitting pieces together.How can I join these individual pieces 12" by 12" tiles to make longer or larger planks for my project please?
The right way: buy a bigger piece of wood.
For very limited, low-stress uses, the biscuit joiner as mentioned by @Nathan might work well enough - as might a Tongue and groove joint, double-groove and spline joint, or lap joint. While a scarf joint (various types are available) is strong, with such short pieces of wood you probably don't have enough wood in each piece to support two scarf joints - not to mention that they are fiddly as heck to cut, and make buying larger planks look a lot more sensible. A scarf joint needs to have a long tapered form to have strength.
For more robust uses, without buying a bigger piece of wood, you could double the thickness and glue them in an overlapping manner (like a "running bond" in bricklaying.)
If you're joining two pieces of wood along the length of the grain, the glue bond can be stronger than the wood itself.
Butt joints are the exact opposite. If you need to join two grain ends, you'll want to reinforce it with biscuits or dowels (or possibly something even fancier like dovetails).
Without knowing what you're attempting to make, my suggestion would be to stagger the "planks" so that they make a solid 6" joint on the sides and reinforce with dowels on the ends.