Have you ever used toys or had sex ?
I ask, because that can give you a big clue about your preferences.
Especially if you've never had sex, I find that girth matters much more than length - it makes more of a difference and if you still have a hymen to some degree or even just nerves, girth I feel is a bigger factor.
Length-wise, I think 6-7" is an okay starting point, but it really depends on your anatomy. You "stretch" more when aroused, but individuals have different preferences and accommodations.
Definitely get lubricant, no matter what !
There are a lot of factors, so I would take any experience you have now into account before making a choice.
Material, g-spot curve... how much curve... in this regard it might be best to buy some cheap things first if you have no clue what will suit you best. That way you can try out shapes and textures before getting something more expensive.