How do I seal a gap between concrete step and wood at outside of house?
The best solution here may be to clean up that area of all debris, let it be fully dry and then apply a bead of qood quality exterior grade caulking material. Since some of the siding looks like it will need re-priming and painting make sure that you use a paintable type of caulk. 

Note that there are various types of caulk. I suggest that you use one of the types that will stay flexible and pliable so that environmental condition with thermal expansion and contraction do not cause the area to open up again. Some of the older types of caulking or sealers that came in a can can dry out and get hard in a year or so and are thus not recommended.