Ok i play rhythem guitar and learnt my self, so i only really know Chords, I know i should learn scales but its been too hard to find the time, but our bassist as left the band, and we want to be a 3 piece and i want to know, how easy it would be for me learn bass, for example, Is there anywebsites that show Bass Chords, and Bass scales or are they the same with Guitar scales, also, How would i put a bass line with a guitar playing with me, so if the guitar is playing for e.g C, Amin and Fmaj7, would i as a bassist play a c scale??? – I hope you understand me, and all questions welcome 
Bass play of 6 and 1/2 years here.
This isn’t too hard.
Transition from rhythm guitar to bass isn’t nearly as difficult as lead guitar to bass. At least with rhythm guitar you have some knowledge of chords and rhythm.
If the guitar is playing a C chord, you will play the root of that chord on the bass, C.
So instead of playing the C chord on the bass, just play the note C. The guitar should be higher, not only in pitch but also in tone because the guitar has the chords. The bass connects the rhythm guitar and drum line together.
Get a basic beat going with your drummer, say 4/4 for simplicity’s sake.
Normally, you would take the bass and have it lead up with chords before a guitar part is written, but you don’t have enough experience with bass, so we’ll change that out for your guitar. It will all sound the same when you throw it together, so don’t worry.
Once you’ve got your drummer continuously following a 4/4 beat, synchronize the chords and rhythm you’ve got with your guitar to the beat of the drum.
Now, take the bass, and play just the notes of the chords you’re using on your guitar. Play them at the same time as your guitar is playing the chords.
Now, as you’ve got that written down, go back to your guitar and do it again, only this time try to add a bit of something to the guitar part. Instead of strumming a chord, do an arpeggio with it, and then go back to playing again. Something like that to give it a little variation.
The bass line will stay the same, your rhythm guitar will follow with it, and occasionally your rhythm guitar should fluctuate a little bit for flashy lead purposes. Just a little though, not a whole lot or it’ll ruin it. A very small amount.
There you go. That’s the bassics. (pun intended).
There are more technical aspects to writing, such as fills, solos, etc. but you don’t need to get into that quite yet.
Just take your time, it will all come to you with practice and thought. Have patience. If you don’t have patience while writing you won’t write anything good. Don’t force the sound. If you’re not feeling creative, just put it down for a bit, and come back to it later.