There are a lot of skills and spells you can train as you progress, each of which costs money. When you can afford to, you should train all the abilities that your class trainer offers. If you're completely broke, it's fine to put off upgrading abilities you rarely use for a level or two so that you can upgrade your most-used abilities. If this happens, you should ask yourself whether you are spending too much money on buying unnecessary equipment upgrades or leveling production trade skills. Be sure to save enough for class abilities and riding training before spending money on other things. This assumes you're earning money at a rate where buying skills makes a difference at all. If you have a hundred gold from two gathering professions by level 20, those skill ranks costing a fraction of a gold won't make any difference to your purse.
Footholds into enemy territories can be done as you grow in levels and should be completed before reaching Level 120. Take the time after completing your first zone to do your first foothold, or you can choose to complete one as soon as you gather your 100 War Resources which will prompt you to return to your mission table area where the first foothold quest can be obtained. Similar to how your initial zone selection was largely irrelevant, this selection is, as well. Simply pick one and complete it to earn your Mission Table second champion. Send your champions out on missions, recruit troops, and then head back out to quest in your second zone selection.
Be cold-blooded about admitting that you've taken a bath on an item. If you bought that sword for 5g, listed it for 10g, and the listing cost is 2g50s each time, after two times it had better sell just to break even. Once you hit that point, don't keep listing it over and over in desperation trying to make something off the AH. D/E it, or vendor it, and move on. Lesson learned. Don't get trapped in the fallacy of sunk costs.
The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold. If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.