Best Airport to take the train .. Gatwick or Heatrow ?

When I arrive in London, I need to take the train to Cardiff .. will I be better to go to Gatwick or Heatrow ? .. any tips (sorry if I mispelled the names)

also, when in the train on a long trip liek that, do you keep your luggage with you, or do the train personell have a storage for it ?

Null

Personally, I would choose Gatwick; change of trains at Reading.  If you fly into Heathrow, you'll be directed to the bus to Reading for the Cardiff train, or you could "double back" into London on Heathrow Express and change at Paddington onto the Cardiff train.

You keep your luggage with you on the train - or rather you can keep it on racks at the end of the carriage, at least on the Reading -> Cardiff.

shoudl I try to get tickets ahead, or can one just buy them at the station ?

You can buy them at the station;  if you know which train you'll be catching (but you can't predict that from a flight) there might be bargains to be had if you book 2 weeks ahead ... and it's sometimes worth looking at Britrail fares before travelling because they're a lot lower than you can get in the UK.  Probably not worthwhile (perhaps not even possible) for a single journey to Cardiff though.