http://www.wowpedia.org/Widget_API#StatusBar
WoW offers a simple widget you can use.
lua Code:
local b = CreateFrame("StatusBar", "MyOwnStatusBar")
b:SetWidth(100)
b:SetHeight(20)
b:SetPoint("CENTER",UIParent,"CENTER",0 , 0)
b:SetMinMaxValues(1,100)
b:SetOrientation("HORIZONTAL")
b:SetValue(75)
b:SetStatusBarTexture("Interface\\TargetingFrame\\UI-StatusBar")
b:GetStatusBarTexture():SetHorizTile(false)
b:GetStatusBarTexture():SetVertTile(false)