Not sure how to answer you.
What year truck do you have? What are you intentions for its use - eg offroading vs blacktop, bling vs practicality etc..
Yes you can fit 18's - lots of people put on 20's
It will mean a change in tire size depending upon what you want to do with your truck.
Adding a lift will obviously give you more clearance but again it depends upon your application.
More details would help.