If your not comfortable going nude (which = no tan lines), wear either a bathing suit or underwear. Both of them will cause tan lines, its inevitable, so just choose which one will cause less noticeable lines. Some people wear a small towel over the boobs and a tiny underwear, some people wear string bikinis and undo the strings and leave the suit on while they lay down.
The best (and safest) way to no lines is the spray tan, because then they could spray to all the missing spots. Just call up the place and ask them what they recommend you wear if you choose that.
Either or is up to you. It depends on how exposed you'd like to be on a tanning bed.