The only way to get a straight grind line is to practice, practice, and practice some more.
Some guys grind totally free-hand, some grind free-hand but hold the spine (back edge of blade) on the work-rest of their grinder, while still others use a grinding fixture.
I have one of those fixtures, but I grind free-hand now. It was easy to grind blades straight at first, but you're somewhat limited with it. I'm not saying it's not a good tool, but I think if you can grind free-hand you have a great deal less limitations. BUT, I know there are many many custom makers that use them with great success.
With a big-blade, the fixture will make a curved grind along the edge that's the same radius as the cutting edge of the blade. Whereas most free-hand grinds will go from the cutting edge to the spine.
If you don't know of any makers that might let you come see how it's done, I would highly recommend you get some videos.
Loveless has one that's really good. It's kind of mind-boggling for a guy like you just starting out, because he's using so much equipment. But, you can get to see a blade made from start to finish, and see how they hollow-grind in his shop.
There were two tapes made by a fellow using a Blademaster that I used to have but they were "borrowed" permanently.
Your best bet is probably to order a catalog from Centaur Forge Ltd. (414) 763-9175.
They have many books and videos on knife-making. And no, you don't have to be a bladesmith to make use of their catalog. I'm not.
I think the best test to see if this is something you want to keep trying at is to order either a kit knife, or make one out of good steel using a file for the stock removal. It will save you a lot of money if you change your mind.
Best of luck
Nick