Wǒ zhǐshì suíbiàn kànkɑn.
我只是随便看看。
I'm just browsing.
Notes:
我(wǒ): I
只是(zhǐshì): just
随便(suíbiàn): casual, informal
看看(kànkɑn): to look around
Example:
A:你要买什么?
Nǐ yào mǎi shénme?
What do you want to buy?
B:我只是随便看看。
Wǒ zhǐshì suíbiàn kànkɑn.
I'm just browsing.