Which one do you have? Here in Germany I have never seen a Heller 1/200 kit labelled "Spanish Galleon". In fact there are 4 variations and they are all based on the same hull. There is the one with the oars named "Galion", the one with the 4 side turrets (2 each side)named "Stella" and the one with 8 turrets named "Corona" or "Elizabethan" (same kit, different box, latter suggests an English ship of course).
IMO the "Galion" is quite, well, almost... buildable for a beginner, but "Corona" and especially "Stella" are too fiddly. All three require you to glue a strip of plastic to a recess in the hull -hard to get that properly in place - you might try to slide it in, secure with tape and then glue it from the inside.
The biggest problem with these ships is that there are no alignment tabs or pins for most of the superstructure parts. You actually have to dry-fit (with tape) everything before you can be sure where exactly everything is supposed to be placed.
If you have the Stella, there are those parts to heighten the hull sides (32 and 33) - I´d glue them on before joining the hull halves to get more stability. Another pitfall: the instructions point to a rim on wall 40 where you are supposed to glue the deck part 39 - that´s wrong because the lower deck (made up from 30 and 31)already sits at this height. Then you just glue the stairs 59 and 60 to deck 39 and glue the whoöe thing to wall 40 with no alignment aid... The more I look at this mess of decks and stairs the less I can recommend "Stella" as a beginner´s project.
As other members have said, there are other ships on the market that are MUCH easier to build. Revell Bounty, Mayflower or Golden Hind are far easier (you can even get the Heller Golden Hind because it IS the Revell model), so if you really want to build a neat ship model in a short time without frustration and don´t mind the extra cost, you should consider buying one of these.