Hi, welcome to PF. Just to let you know, there is a homework help section for these kinds of questions. If you have more homework questions in future, you'll get the most helpful support if you ask them there.
Ok, so for this question you could just insert your values of [itex]a[/itex] and [itex]s[/itex] into whichever formula seems to fit, as is too often taught in schools, and in doing so stumble across the correct answer. But you learn much more if you ignore the formulas and work out the answer step by step.
Here, [itex]u[/itex] = 0 m/s, [itex]a[/itex] = 9.8 m/s[itex]^2[/itex], [itex]s[/itex] = 10 m and [itex]v[/itex] is the unknown. (At this point you could simply use [itex]v^2=u^2+2as[/itex], but please abstain from doing that until you're satisfied you understand where that formula from.)
The most intuitive way to visualise these types of question (in my opinion) is to introduce time. It's just easier to visualise what will happen to the ball/projectile over time, rather than over distance.
I'd start by expressing [itex]v[/itex] in terms of [itex]a[/itex] and [itex]t[/itex] ([itex]u[/itex] is zero so you can ignore it. Zero plus anything is zero.) Next: given that your formula contains the initial and final velocities, express the average velocity of the ball over time period [itex]t[/itex]. Next: Distance = average speed × time. You'll end up with a formula that starts with "[itex]s=...[/itex]". Finally, let [itex]s[/itex]=10 and solve for [itex]t[/itex].
This is really an unnecessarily long method, but by thinking several similar problems through in the same way you'll understand how to solve them. Next, derive for yourself the kinematics formulae listed in Georgepowell's post. Then you will have "earned the right" to cheat by using the formulas straight off the bat!
- m.e.t.a.