Is your difficulty the export itself, or is it getting a table of values?
In your example, the value of s should be an InterpolatingFunction. That's just like any other function that can be applied to arguments. So, if you wanted a table of values for your range {0, tf}, then you could just evaluate s for values in that range spaced at whatever intervals you want. Like this (let's say tf is 10 and you want your spacing to be .5):
Table[{x, s[x]}, {x, 0, 10, .5}]
Or if you want to specify the number of samples (e.g. 35) instead of spacing:
Array[{#, s[#]} &, 35, {0, 10}]
If your difficulty is the export, then you need to decide the appropriate format. I assume you'll want CSV, so just use "CSV" as your format when you use Export (see http://reference.wolfram.com/language/ref/Export.html)
