You didn't say if it's a reed-valve engine or a TD/Medallion. If you
have an external tank, it's most likely a TD. Depending on whether
the engine isn't stock, it may need a pressurized (pen bladder or similar)
fuel tank. Assuming it's stock, make sure the tank centerline is
roughly vertically in line with the needle valve.
The plug should be glowing bright orange, not dull red. Red means you
either have a weak battery, or the wrong kind of battery.
If it's a TD, make sure you have one or two squish-band spacers
(they look like thin gaskets) under the glow-head.
Use 25% nitro fuel. Castor-synthetic oil mix is best for these guys.
Cox used to make a great fuel for them.
When you're about to spin the prop, squirt some raw
fuel on the fins above the exhaust port to prime it.
If your glow plug is properly glowing, it should fire on just
that prime, even with the needle closed. Once you verify
that, start with the needle 2 turns open (assuming a stock
Starting them is not that hard. Getting them to run strongly
throughout a fuel tank is the trick.
There are many Cox guides on the internet. Google.