Hey all -- advice needed,

I've always ridden around the city in jeans and/or whatever normal clothes I had on at the time. However in the next week I plan to start commuting one day a week to work by bike. It's 20 miles each way.

I justified buying a Brooks saddle because of this

I've never owned Lycra/bike shorts/jerseys/etc. But it seems like without proper clothes I might hate myself.

Any advice on what to wear -- specifically on bottom? Should I get padded shorts? Can I wear shorts under normal pants? Are liners vs shorts better? Is it all overkill and I should just find comfy pants to wear (I'm reading Full Tilt right now and I think she just has normal pants).

Additional info:
I live near SeattleI'm planning on doing much longer rides this summer (~200 miles)
I have a shower at work
Massive thx in advance.