The tortle race on dandwiki has a feature called Shell which states that it has 14 + your constitution modifier natural AC. It doesn't stack with unarmored defense but if I were, for example, wearing leather armor which gives me 11 ac would they stack to 25(14+11) + your constitution modifier or would I just go with the higher one?
Different ways of calculating AC never stack, you choose which one to use
The description of the homebrew tortle on dndwiki says:
Shell. Tortles have a natural armor of 14 + your Constitution modifier. This Natural Armor can not "stack" with any unarmored defense or equivalent gifted by a class. Your natural armor is negated if you gain an unarmored defense from a class.
When you put on armor, you end up giving yourself multiple ways to calculate your AC. The PHB tells how to deal with that situation:
Some spells and class features give you a different way to calculate your AC. If you have multiple features that give you different ways to calculate your AC, you choose which one to use. (PHB 14)
Thus, if the tortle put on leather armor it would have a choice between your natural armor AC calculation:
$$14 + CON\; mod$$
and your armor AC calculation:
$$11+ DEX\; mod$$
Note that armor AC calculations do use DEX by default not CON. Whichever AC calculation is the one you want is the one you use.
If you decided to use a shield that would add +2 (or whatever the shield's bonus is) to your AC.
Note about Dandwiki
The race you are looking at is a homebrew race found on dandwiki.com. It is not official and so you must have a DM approve any use of it. Be very careful because much of the content on that site is very poorly designed and unbalanced and that can cause issues in your game. The official tortle race can be found in the Tortle Package sold on DMs Guild.