Basic Tips? Just about all of the moves
Go to the Marth Q&A thread, the new one, and you can get more answers. If you already know basic techniques and stuff, then you can go more into the spacing of his moves, and utilizing his range in the neutral
One thing, is to grab, and learn how to follow up from those. Grabbing is so good with Marth, as his grab range is long.
Combos revolve around juggling with uptilt and various aerials, and you can usually catch bad recoveries or combo into the dair, which is a spike (Ken Combo yeet).
This is really just scratching the surface. I'm sure more veteran players will be able to phrase it better
Good Luck!