The logic for picking cards is somewhat complicated. For a simpler (but less detailed) spreadsheet view, look here. Generally, plan to finish the first floor with 3-5 cards added to your deck.

Your top priority is to improve your offensive capabilities. The main offensive cards you want to find at this point are, in order of priority:

Whirlwind, Pommel Strike, Cleave, Fiend Fire, Inflame, Body Slam, Rampage, Heavy Blade, Headbutt, Iron Wave, Uppercut, Spot Weakness, and Twin Strike.

If you are past floor 4 and haven't seen any of the cards above, try to at least get one of the cards below before fighting any elites:

Armaments, Clash, Clothesline, Flex, Bludgeon, Hemokinesis, Thunderclap, Pummel, Infernal Blade, Sever Soul, Carnage, Wild Strike

For this strategy, I don't recommend ever any offensive cards on level 1 that are not on these lists (it is strong to, for example, pick Searing Blow on the first floor and pursue a Searing Blow strategy, but that is not covered by this guide). You generally want 2-3 of the above cards, which will comprise a large portion of your damage until the end of the game.

Defensive and utility cards are less desirable than offensive cards until you have enough offense, but again, there are a few that you should be looking for. Pick mostly from this list, in order of priority (a maximum of one copy should be picked for each card with a *):

Battle Trance*, Shrug it Off, True Grit*, Disarm*, Shockwave*, Inflame*, Offering, Reaper, Impervious, Double Tap*, Flame Barrier, Evolve*, Ghostly Armor, Feed

Finally, there are a few rare cards that you should always keep an eye out for. These are your win conditions:

Limit Break, Demon Form, Barricade

After you feel comfortable with the rest of your deck, you should take the first one of these that you see, but no more. Afterwards, you will build your deck around the one you choose. Be careful when picking these, however, as they are likely to be dead cards in your deck until at least the boss of level 1.