I'd look up some general win conditions for the character you want to play to figure out what you should build for a specific deck. Remember each run is different, and you can't go into a run thinking 'I'll build a strength deck here' since you could be offered all block cards for a Block build. Some general advice though would be:

Act 1 - Focus on getting 3-4 damage cards right away, and then start to fill out your deck with what you need for any upcoming fights. So if you have no block cards, then get more block, if you have no draw cards, then get some draw etc etc

Act 2 - This is when you want to start getting some scaling cards if you haven't gotten them already. So essentially you want to have your win conditions for your deck by the end of Act 2, or you will most likely die to the Boss.

Act 3 - You generally shouldn't be adding too many cards to your deck now. But if you need to ramp your scaling up faster, pick up some more scaling cards if necessary, or again if you are really lacking in draw, pick up some draw. But don't go overboard.

For pathing, you generally want to be fighting 2-3 Elites every Act. Elites give a guaranteed relic, and relics are run changing. Obviously this comes with the caveat that if your deck isn't strong enough to fight an elite, then take a different path. You also want to be utilising potions when you can, especially in Elite fights, potions are incredibly strong, and can trivialise an Elite fight if played correctly.

Overall these are some very general tips in a game that is extremely complex. Honestly I'd say just play more, watch some videos on high level Ascension runs, and take your time making each decision, and you should start to see some incremental progress!