okay so i got my uni for christmas and really like the idea of being able to hop up all these really high things 
but ive really got no idea on how to get higher except keep practising?
also i think i have to do sif hops to get higher so i can tuck? but i have no idea where to start with that 
anyone got any pointers or anything like that?
just start riding and hopping around on flat pavement and doing as many things as possible sif. this will let you get comfortable enough with it in order to jump up higher obstacles with tucks.
so do SIF till it feels just as natural?
That would be a good start, yes.
What’s your sidehop right now? I wouldn’t (and didn’t myself) learn SIF until I was getting around 35 or 40 cm consistently. I think it doesn’t matter until you get around or past that point imo, plus it’s good to learn how to be able to do average sized hops SI.
my SI hops are at about 11" which i think is like 25cm?