Building items/learning about items?/Tips for learning other lanes?

Hello, this is my first post here. :) I'm only level 17 and league is the only moba I've ever played. I generally play mmos and focus on one class per mmo and I've found that's translated into league a little bit. All I play is support, but specifically Nami. I've tried a few other supports (only tanky support I've tried is Leona though, but I do enjoy her) and enjoy the role as a whole.

I'd like some tips on other lanes though. I tried Lulu when she was free and since she can go a lot of different places, I bought her. So she is someone I would like to learn. I also enjoyed playing Shyvanna, but I get nervous trying out roles other than support. I feel like I'll just let the rest of the team down if I play in a normal game any other role lol. Part of the reason I bought Lulu for other roles is that I could play her in support to get used to her skills and such and then just play her differently if I play her elsewhere. So for the tips of other lanes part... What would you recommend I try? Laning tips in general, champs, etc. I picked up support because it's a duo lane so solo laning is also something I'm nervous about.

For the build question it has to do more with Nami. Almost all my games are Nami from like level 7 to where I am now. I do play her pretty aggressively when I play at my elo, but more passively when I play with my friends. (They're high silver to mid gold, so not exactly incredible but definitely above my level.) When I'm on my own I usually build mostly AP since I am pretty aggressive, but I don't really know how to build for when I want defensive and that sort of thing. Honestly when I build something with an active ability I usually forget about it until it's too late to use it lol (that's something I know I really have to work on)

What items do you think I should build, and are there any tips for remembering to use items with abilities?

Is there anything I should be doing to learn more about different items or will it just come with time?

Thanks for taking time to read this/helping :)