Why not start with the basics?
Both the battery terminals have to be tight on the battery post. There should be a little 10mm nut & bolt on the terminal so push the terminal onto the post and then tighten the nut up until the terminal doesn't turn or move up and down when you tug on it.
Next, check that the battery is charging up. Start the car (jump leads) and measure the voltage across the terminals using a volt meter. The reading should be 14.5v or thereabouts. If it's not that high, you have a problem with the alternator.
If it is charging okay (14.5v) turn the engine off and go have a cup of tea. Come back later and measure the voltage across the terminals now. It should be 12.6v or more. If it's less than that (12v or less is not enough) then the battery needs charging.
Take the beast out and charge it on the bench at the lowest ampage your charger can deliver, for 12 hours. Turn the charger off .. go have a cup of tea.. then measure the voltage. It should read 12.6v or more, because you were just charging it. If it's less than that, after 12 hours charging, the battery is knackered.
If it charged to 12.6v stick the battery back in the car and tighten the battery terminals. That should be sorted then... but if the battery doesn't hold its charge, either it is knackered or it is being drained.
If it's being drained you need to check why .. and that's where you have to measure the ampage in the circuit. If you have any current being drawn, you can identify what's drawing it by taking the fuses out one at a time until you identify what component is still "on". Best place to start is always the radio, expecially if you have a different ICE than standard.. nothing, apart from the alarm/immobilisor should be drawing current when the ignition is off... and the alarm/immob. should be negligible current.
My best punt is your battery is not being charged up (loose connectors) so it goes flat. I doubt it's a knackered battery as it's quite new... though a battery kept permanently in a low state of charge will become damaged, same as a very old one.