"Like how do people create arpegios and bases and synth hits etc - they must use something -"
They use synths.
Yep, ok understand what you are asking for now.
Well as I said and a few have said, you don`t need to make sampled bass sound hits or synth hits, to then sample and then sequence.
You can just use Vsti`s
It sounds like you are just about to truly begin your journey into proper production, but basically to make and use synth sounds, you are better off using synths.
There`s a forum called KVR/Vst which is a great resource for production advice, tutorials AND free vst synths and effects (which you can use in your sequencer)
Also the free vsti thread sticky at the top of this part of the forum is a fantastic resource for vst synths.
First you need to find out how to use and indeed what a vst instrument is.
Go here
http://en.wikipedia.org/wiki/Virtual_Studio_Technology
There`s many explanations AND a nice big bunch of follow up links at the bottom.
Also here is a nice link explaining basic sound control
http://en.wikipedia.org/wiki/ADSR
When beginning playing with synths and making your own sounds, you want to really start with a 1 or 2 oscillator synth. And then go from there.
Good luck, and also have a good dig in this section of the forum as there is tons and tons of good advice, tips, info and banter.