I was told to regrout my shower. I was wondering if there is an alterative to it. I was thinking of using caulk
For grout, I was thinking of using pre-mixed grout such as "SimpleGrout" which is avaliable at HomeDepot.
Any type of filler other than grout shouldn't be used to fill the joints between tiles. You will need an approved cement-type grout either sanded or not. Sanded is called for when grout joints are wider than 1/8- 3/16 inches.
You can also use a two part epoxy-type grout which is more durable and mold resistant, but is much less forgiving to use.
Any caulk or silicone won't last very long in a wet environment and is difficult to apply to just the joints.
Re-grouting entails removing any cracked or damaged grout with a grout saw or similar tool. By hand it is a tedious and slow job. There are power tools with attachments for grout removal that will shorten the work, but attention needs to be had in order to not damage the tile.
Once grout is removed from the affected area and the debris vacuumed out new grout can be troweled into the joints. It is easiest to color match the grout if there is grout left over from before. A similar color from the store may not match exactly.
Seal with an appropriate grout sealer when dried and before using the shower. (1-2 days).
If the grout in the corners has cracked, it will most likely crack again if you re-grout the corners. You must have a very slight amount of movement between the walls and the floor. Grout has no flexibility.
The grout is not a water barrier, so you don't need to do anything unless you are bothered by the looks.
I would caulk the corners only with a color that looks good. Use something made for wet areas that is mildew resistant.
Note that this recommendation is for the corners only. Never use caulk on the flat areas.