Hi cg, welcome to the forum.
Your questions are typical for someone new to exploring precious metals and the answers will depend a lot upon your personal goals and circumstances.
What to invest in will depend upon how much money you are looking to invest and what facility(ies) you have for storage. Silver is more volatile (price) than gold, so there is more opportunity for growth at the moment (
gold-silver price ratio is still pretty high and I expect it to go lower as the metals break out to the upside). But if you have a good bit of money to invest, you'll find that storing a large stash of silver can become a logistical issue. Gold is better (logistically) for larger investments.
When considering the unit size to buy, I prefer to think about the liquidity issues. How easy will it be to sell it eventually? Larger sized units (100oz silver bars or larger for example) may be more difficult to sell (to private buyers anyway) if the economy is down. I like 1oz coins/rounds/ingots for gold (smaller units are really small and I worry about losing them). I like silver in rounds/bars anywhere from 1 to 10 ozs.
The storage issue is something you need to figure out yourself based upon your circumstances. Some people store privately (either in a safe, or hidden somewhere), some prefer a safety deposit box (I don't - banks can go on holiday among other issues) and some prefer to use a vaulting company. The right answer for you will likely depend upon the area where you live (is your house secure or likely to be robbed?) and how much you are wanting to invest (not worth paying a vaulting company to store 10 ounces of silver).
Best place to buy? Since you mentioned Apmex and Ebay, I assume you are in America. There are a lot of online dealers you can choose from (see our
Marketplace forum for member reviews of most of them). Apmex charges a bit more than most, but they have a great reputation for customer service and delivering on time. Ebay is ebay - caveat emptor.
I'd suggest searching around to see if there is a brick and mortar coin shop in your area. They won't necessarily offer a better deal on price than online dealers, but it's good to have a relationship with them when you want to sell some metal or take instant delivery on a purchase.