If I install elementary OS on my pc alongside with Windows 10 by creating partitions manually, will I be able to choose what OS I wanna boot at computer startup?
Yes. GRUB will install itself into the boot part of your primary disk, so you can choose at boot which OS to start. Alternatively you can choose the mainboard boot menu with F11 or the similar button. If GRUB is not working as expected try "grub-repair" (just google for it) You should install Linux after Windows, otherwise windows will overwrite the boot sequence. I would suggest to start Linux Live Mode and use "gparted" to shrink your windows partition.
