Here are some queries on StarCraft modding:
1.) Is it possible to edit the GRPs of the menu (unit selection like 'Train Marine')
2.) Is it possible to give the Terran Dropship a weapon?
3.) Is it possible to give the Terran Dropship spells?

Thanks for answering the noob!

1 - If by menu you mean ingame menu, you're looking for cmdicons.grp I think. The main menu, with the campaigns, is smk files. These are similar to grps but you need the old rad tools for them.
2 - Yes, but iirc the dropship has some odd behavior associated with it through hardcode and weapons.
3 - Some spells are hardcoded, like cloaking, but others you can probably add without issues. It will need a spellcasting animation.

Thanks! I still have one question? How do you edit the wireframes?

Wireframes are also a grp. I forget the exact name though.

You can look through Patch_rt.mpq, BrooDat.mpq, and StarDat.mpq to find files like these. Generally, you'll want to look through Patch_rt, then BrooDat, then StarDat in that order for relevant files.