Two resistors for potential divider (15v to 5v), 08M, readadc command.
Power 08M via 7805 voltage regulator (plus associated input/output/decoupling caps).
Display, depends on what you want to show.
AXE033 driven with serout is easiest. 1 pin.
3 X 7 segment display driven by 4026 counters very easy.
Wasn't this exact question asked a few days ago?
GCSE coursework by any chance?
A potential divider which converts 15V to 5V fed into a PICAXE ADC input, an accurate 5V supply rail, a means of display, and a few line sof PICAXE code. So that's two resistors, a PICAXE and an AXE033 serial LCD module for the most basic PICAXE solution.
For more accurate readings you'll need an accurate or adjustable potential divider and/or an accurate power supply or a reference voltage to allow PICAXE software to compensate for any inaccuracies and fluctuations.
Depending upon the voltage source you may need to buffer the input volatge through an op-amp and that can add complexity if you need to go down to 0V or up to 15V.
You could of course just buy a digital panel meter which would probably only need a power supply plus single resistor to configure and calibrate.
Just buy a cheapie DMM- these sell here in NZ for just ~US$3-5! However the open circuit voltage of a lead acid battery is additionally not a valid measure of the energy within! Older ones develop high internal resistance & no longer supply decent currents. You have to test under load normally.