What makes more sense?
If i level up only my main class and do a single class character?
Or is it better to choose a second one after the first level up?

The choice of what classes to take is a personal one, but you should consider that a second class should complement or enhance the first class in some way, whether it be stats or feats or some other consideration.

Aren't there guides online for Pathfinder monster builds?

To be honest, nothing makes sense for a paladin. They're dedicated holy warriors with a single purpose. That being said, taking a couple of classes as fighter gives you extra feats and can be easily justified, being another warrior class. Personally, though, I'd never dual class a paladin.

Really it's all about personal taste. play what you enjoy
Sinlge class characters (Especially spellcasters) are a bit more powerfull multiclass are more versitile.
Powergamers add in bits of other classes just for special abilities to buff up their character..

I personally wouldn't multi a class that was already essentially a multi...like Magi and inquisitors. Except maybe preige classes

I don't think paladins have that many choices for multiclassing. Src/Pal can work, as can Brd/Pal, but those are usually with Paladin as the secondary class. I'm not sure which alignments are allowed for Paladin in Pathfinder, but if Lawful Neutral is acceptable then Drd/Pal might also work with Feyspeaker since they can use Cha as their primary casting stat.

Classic multiclasses include Fgt/Wiz for Eldritch Knight, Arcane caster/Rog for Arcane trickster and sneak touch attacks(melee and ranged), Drd/Mnk for wis bonus to wildshape AC, fgt/rog for bonus feats to dual-wield, Rng/rog for ranged sneak attacks, clr/fgt for combat feats and full bab. There are many many others.