What sort of opticians doesn’t offer a first time wearer some sort of advice? They don’t deserve you, go elsewhere with a good wide range and try everything on. There’s not a good hard and fast rule to choosing, as fashion dictates as much as face shape. The chart posted above recommended oval frames, what is this? 1994?
Listen to the dispensing optician in the store, they don’t know you or your style so will give a neutral and unbiased opinion, if you think you look like a **** then tell them, if they’re any good they’ll be able to quickly narrow down the massive range into a handful of suitable styles.
Specsavers usually have a decent range at sensible prices, vision express are similar but offer brand names if that means anything to you, boots are hit and miss in their stock ranges, independents should offer you plenty but generally at a price.
Don’t decide against a frame until you’ve tried it on.
Have a budget in mind
Don’t rush
Ask them to keep stuff aside for an hour while you get a coffee
Buy an anti-reflection coating, those who tell you otherwise are wrong.
Good luck
IAADO