Some help would be appreciated on where to start as a guide when writing, I'm looking at playing bass in a post rock style band.
I understand starting with roots and then going to 5th and 7th's and jumping octaves.Thats literally bass lesson 1 and I'm stuck from there.

Just don't know where to go from there really. The genre isn't very structured so don't know if I would be better starting with a melodic bassline from a scale (probably based on whatever chords are being played/ ringing notes/ key of general atmospherics) and going from there. Rather than standard rock that focus' around the root and plays pretty close to that.

I'm pretty new to writing full stop so really don't know how to approach the whole thing let alone in a complex genre. I want what I do to often drive the direction of it as well as subtly compliment it. Thing Mogwai where sometimes the bass in the background and others its the whole melody.

Any help is welcome ,would rather not get a slagging off on why i own a bass but still need to ask for help with writing. Thanks
Hey man, I'm a bassist myself. Some advice I'd give you straight away is to invest in a scales book if you want to expand your riff creating prowess. A scales book is like a roadmap that shows you exactly where to go - it may also be worth your while to find out the structures of various arpeggios (major, minor, 7ths etc) as well.

Although all this sounds boring, I assure you once you have done a bit of the theory the rest becomes much easier and funner to do! Knowing some new scales will make your soloing much more complex too. Speaking from personal experience, when writing a riff I would first have a mess on with the bass without deciding a key. When you find some interesting melodies and rhythms, then use the music theory you have learnt to expand upon that and find where to go from there.
One of the things I miss in a lot of bass players is the ability to play with the drummer. This can be as basic as hitting notes at the same time as the bass drum, or as complicated as following a tom-groove, both rhythmically *and* melodically. With that I mean that you follow the contour of the pitches of the toms with your bass, this can create a great effect.

Melodically, one of the things you can do is smoothing out chord movements. Say you have a C major chord which moves to a E minor chord. During the first chord, you could be playing around the C note, and just before the Em chord hits, you hit a D note. This way, you smooth out the movement from C->E with C->D->E.
^ Excellent point about playing with the drummer

I'd also reccomend at least basic theory... learn how chords are formed, chord tones, scales, how to use passage notes, chromaticisms... all that will help in creating more interesting bass lines