How to fill a crack on my concrete porch?

Check into Sika. I've seen it at Home Depot.

Also, they make a concrete filler that comes in a tube like caulking you can fill in.

There are many products and methods to go about patching or repairing a cracked concrete patio slab. The main options are:

• Caulks & Sealers – inexpensive, easy, non-structural (i.e. has no strength), surface patch of uncertain long-term durability. Only for cracks less than 1/2″ wide.
• Concrete – inexpensive, semi-structural, repair to full depth of the crack. Best for cracks 1/2″ or wider due to difficulty of forcing concrete into a narrow crack all the way down in a 6 inch thick concrete slab. For best results, the crack should be undercut with a chisel to form an inverted V notch for maximum holding power at the “cold” joint between new and old concrete. Concrete is relatively weak in tension and can’t prevent slab movement if the foundation is less than 100% stable.
• Polymer Resins – expensive, deep penetrating, structural repair which forms a permanent chemical bond with the concrete slab, gluing the slabs together. Compressive- and tension (i.e. “pull apart”) strength of 4,000+ psi exceeds that of concrete to prevent the slab from moving. Slightly less rigid than concrete for stress relief. Epoxy and Polyurea adhesives are example materials.