First you need to evaluate the costs of your race in terms of req/power and their eco aswell, generally orks/nids and eldar with 3 DA or 1 DA + shees will have alot of req so generally you can instatly build 1 node + 2 or 3 gens depending on your next purchase.
For example supression teams always cost more power than req in terms on how much time it requires to get that amount, every setup team is 30 power and between 225 req and 250, so you want to put a node as soon as possible and 2 gens, a third one asap if you plan to get alot of upgrades too, on the other hand a squad like ASM that's 450/50 will take more time to build up the req needed when you take into consideration the investment of req for generators, and usually the right balance taking in consideration a death of 1 model it 1 node + 1 gen.
Then the more the game lasts the more req becomes important, simply because of upkeep and bleed, if you managed to keep all of your units alive your popcap will increase still the max as you go on in purchasing new units, that means that you will eventually bleed a decent amount, and usually the power bleed is pretty much ignorable what you will be missing most of the time will be req regardless of race.
As far as unit purchasing goes, that's entirely up to you and your playstyle, there are people who like t go with an heavy T1 and purchase 2 or 3 units in this tier and alot of upgrades and there are people who do a very light T1 and play an moderately heavy T2, and then there are people who try to count on their T3s, so that's entirely up to you on how you want to play the game, but generally getting atleast 1 t1.5 unit (T1.5 beign the T1 units that cost power to get) is the best idea as it grants you more units for map control aswell as it will most likely require your opponent to purchase something to counter it.
And then, the wargear is the same, play around with it, see what you like and what fits in different situations, usually you want to purchase it you to strenghten your composition or to specialize your hero into something that might be tiyng up,supporting or capping around the map.
I hope i expressed my toughts on this decently enough
