How should I tell my mum I need bras?

okay so I've asked her about 4 weeks ago and she said yes but I think she forgot to go out with me on the weekend because we didn't.. coz she got called into work and on sunday's all shops r closed... so when i wear singlets under my pj's it shows .. u kno and i really think i NEED themm.. as in, proper ones.. but like I'm too nervous.. nd embaressed.. maybe i could bring my friend with me to bring up my confidence.?? :s I'm 13 btw.