Cyrus 07,
First of all, don't ever apologise for asking questions, the gents on this forum have a wide variety of knowledge and there is always someone ready to help. Everyone of us have asked questions at one time on this forum when we needed assistance from our peers.
I am going to disagree with some of the statements made in response to your post but will try and justify with some explanations based on personal experience.
The code (not the wps) that most pipelines are constructed to is API 1104.The WPS (Welding Procedure Specification) is the document that has all the information required to produce a sound weld (including the type of electrode ,direction of travel, amps, volts etc,etc).The predominant electrode used around the world for cross country pipelining is the cellulose electrode (XX10...eg 6010,7010, 8010).The 7018 electrode is a low hydrogen electrode that is used predominantly in refineries, power stations etc, usually vertically up.However, dependant on the service there is a variety of different combinations of electrodes and directions of travel and these will be detailed on the WPS and when/if you test you will perform a weld as per the WPS. Some I have worked to over the years - 6010 VD All - 6010 VU All - 6010/7010 VD - 6010 VD/7018 VU - 6010 VU / 7018 VU.
I was a pipewelder (GTAW/SMAW) for 5 years when I crossed over to pipelining. My mentor, a grizzled old veteran told me the first day if I wanted to be a good "stovie" I had to forget everything I had ever learnt about vertical up welding and start again. If you have vertical up knowledge/experience it is always a good fallback option when there are no pipelines running, but if you want to concentrate on pipelining practice nothing but vertical down.
When you are running bead on a pipeline you will have the aid of a helper who will change your heat up and down for you as you are welding dependant on your root gap, so if you practice,practice,practice until you can put a perfect root in with no change in heat you will know you are almost there.
Start with a 1/8" electrode and when you have got that running nicely - root, hotpass, fill+cap, move up to 5/32" for the fill + cap and then when that is going well move up to 3/16". The bigger the electrode, the harder it is to weld so start small and progress up as you feel confident.
Hope that helps,
Good luck,
Shane