Sheltered Kindred here!

Can anyone help me with some info about how to acquire a suit / formal attire? I grew up sheltered as the titles suggests. I have no experience ever going to a wedding or any formal event. My girlfriend found a dress online as she’s part of the bridal party. It’s a nice shade of pink and we want my attire to have a splash of the same color. Problem is, I’ve never shopped for “nice” clothes. I’m genuinely so overwhelmed and lost. When we get her dress fitted at the tailor, do I look for options there? Are there stores that specialize in this stuff? I have a very slender form and most formal stuff doesn’t fit very well off the shelf. Can anyone offer advice? Much love!