At the early stages, I would recommend building arena. That's pretty easy to do as you can see where you're selling out.
Try to identify specific positions where your team needs help and watch the transfer list to get an idea of prices. The cheapest now are C which would easily put your team over the hump in Div. V. Just don't spend too much.
You can probably get an 18 year old trainee for under $10,000 and you can practice training. Start training and if you screw up, you should be able to sell him for more than you spent.
Most of the game is just common sense and, to emphasize again, as long as you build your arena you'll be fine. At most, you'll spend an extra season in Div. V.