Oh. My. Word!! Do not stress out about being 18 and never been kissed!!! There are LOADS and LOADs of 18, 19, 20+ year olds who are in the same boat and most are attractive as well!! So take that one off your worry list - that is NOT something to worry about! I'm sorry if that is strongly worded but I read about people being concerned about how old they are on this forum all the time having never been kissed and I used to feel the same way. Now I'm older and I KNOW that it is nothing to worry about! It will happen when the time is right and that is at a different age for everyone. It doesn't make anyone abnormal and you are not the only one, or too old, by any means!
If this guy likes you and you are interested in return and he goes in for the kiss you can either kiss him or explain to him that you prefer to take things slow. If he likes you and you show any interest - even just having a good conversation with him - he is not going to care whether or not you kiss him. He'll respect you and be happy to have spent time with you. If he pushes kissing etc. and is 'disappointed', pouts, puts pressure on you then he isn't the guy you think he is.
Your mom is just trying to protect you but unfortunately it has the opposite effect and you are questioning what is right etc. At 18 if you want to kiss this guy then it is okay. Don't let it go further than that though for now. If you don't want to then as I said above that is okay too. I hope you have some good friends that you can talk to about your questions. If you don't have general anxiety then once you've kissed I think you'll quickly get over those nerves. It is pretty natural! Good Luck!