友達を選ぶときは注意が必要です。

友達: friend, friends, mate, pal, companion
を: o, the, to, wo
選ぶ: choose, pick, opt, elect, select
とき: when, time, moment, occasion, ibis
は: a, the, is, am, are, will
注意: attention, careful, caution, note, warning, heed, regard, advice, notice
が: is, there, but, ga, will, however
必要: need, have, necessary, needed, required, must, want, essential, necessity, indispensable
です: is, be, are, am, that, away