I need help with making a character move faster over time.

I’m creating a Sonic game because I already made a Mario game earlier. One issue I have is that I want Sonic to walk slowly but get faster and faster over time. Any ideas?

I really have no idea how to do this and would really like some help. Thanks in advance!

while holding a button down, let the number increase by one until the filter stops it at a set speed (over ##).