How can I get Free Outfit in PUBG?

You can earn BP by playing the game. The amount depends on your kills, what place you got in the game, and whether or not you're playing in squads, duos, or solo.

That BP can be used to directly buy some cosmetics, or cases which have a random chance of giving you different types of cosmetics. The latter gives you items that you can't buy. You can also buy some cosmetics for real money(eg. Streamer skins).

From time to time, PUBG gives out free items for events, so you can wait for that if you don't want to play for some reason.

Cosmetics have nothing to do with the game mechanics and I wouldn't recommend buying them with real money.