Sein ("to be") is probably the most-used verb in German — and one of the most irregular: ich bin, du bist, er ist, wir sind, ihr seid, sie sind. There's no shortcut here; these forms simply have to live in your muscle memory.
Sein is also an auxiliary verb for the present perfect — used with verbs of motion or change of state (ich bin gegangen, wir sind angekommen). If you'd like to compare sein and haben side by side, try the sein vs. haben exercises.
Exercise 1: Sein Conjugation
A walk through every personal pronoun. Type the matching form of sein — bin, bist, ist, sind, seid, sind — and you'll feel the rhythm of the conjugation pretty quickly.
Exercise 2: Sein Conjugation
A second pass with everyday descriptions — moods, places, weather, relationships. Same forms, but now in the kind of sentences you'd actually say out loud.