Need help with starting items

Blade is an aggressive first item, you use it when you can proc it often (i.e. autoattack the wave, autoattack your lane opponent) because it's a ton of AD and lifesteal.

You use DShield when your opponent will not let you proc Dblade often. If your opponent is ranged and (assuming they're good) will contest creeps and auto harass you when you go for last hits, you're going to get more sustain from DShield.

Corrupting Pot is one of those items where you get bullied in a non-"I have 300-400 range on you and will harass you relentlessly" way, so you go for it in the hopes it'll give you the trading power to survive to your item powerspike that does allow you to fight your opponent.