I need help installing a new ROM on my android?

Ok, So I have an android. It’s an HTC Mytouch 3G Fender edition. Its Firmware Version is 1.6, and since it’s 1.6 I can’t get any apps on the app store. I’m stuck using the market. IT’S SO FRUSTRATING! I can’t use any apps that are up to date. I rooted it already. It’s confirmed and everything. So I’m wondering if there is a way to install new ROMs to my phone through my computer? I’ve tried everything possible to try and update the firmware version. I called T-Mobile, I called htc, I checked firmware update in options. I’ve gotten nowhere. So please help!