What deck do I use

Generally speaking as a new player your cards suck. Because of this you'll basically have to load your deck very heavily towards speed in order to be able to win races in training. As a rough guide, you'll want (Assuming 0lb max level cards):
1x wit card (finemo/others)
enough stamina cards to not die on the longest distances you're training for (You'll usually only need one, for long distances maybe two)
then just fill the rest of the deck with as many speed cards as you can

Should I use a Friend card? (Tazuna/kiryuin)

Almost certainly not. A max limit break tazuna is a viable sidegrade to a speed card in your deck, but since you're reading this you probably only have 0lb copies of them. They're pretty useless at 0lb, especially when speed is at a premium this early on.

What stats should I train for?

Speed is king. You need speed to be able to win at all, literally nothing else matters. At some point, if your character has long distance races (I.E. Kikka sho, Spring Tennosho, Arima kinen) you should consider splashing enough stamina to survive these races. Aim for roughly 500-600 stamina for these races in career mode, you can just barely scrape by with 400 or so and a recovery skill. After you have sufficient stamina, put everything else into speed, power and wit as you get good turns for them, with speed and wit taking priority over power clicks.

Why did I lose this race?

Your stats weren't high enough. Either that or you got unlucky.
Generally speaking there will be two ways you lose career races.
It can be because:

• You didn't have enough speed
  this will look like they just failed to overtake during the final part of the race
• You didn't have enough stamina
  Your trainee will suffer a massive speed debuff, they will start falling behind