Stratocasters can be very frustrating...
1. Tune the string
2. Stretch the string by pulling it upwards away from the fingerboard, between the nut and bridge (at the 12th fret is the most convenient place to grab the string)
3. Especially if new, each time you stretch it, the string will come back a little flat. Less and less so until stretching no longer makes the string go flat.
4. Now you know your string is properly stretched.
5. Now bend the string behind the nut (in-between the nut and the tuners on the headstock) - like country players do.
If it comes black sharp - the string is binding at the nut - and you need a guitar tech...