Just installed OpenWRT 12.09 in my router, but became a little confusing about interfaces/bridges/vlans, please help me to understand!

After my first boot, I logged in and setted my root password, so I used Luci to activate my wifi, and took me a little to figure out that I could bridge wifi with lan to get access to the internet. But I ended up with two problems:

1. The DHCP adresses are been provided by my ISP (which is bad since I have to pay an extra tax for each IP)
2. I cant block ssh tries from the internet, so my router is a bit vulnerable.

How should I configure (preferably by Luci, since I do not have the time to learn a lot now) to get:

- DHCP provided by this router to my internal devices, with resolving of device names (DNS)
- My external address (lan/eth0) provided by the ISP modem (so I can't simple block port 68 in this interface)
- Properly configure my interfaces so I can allow ssh only on internal (wan/radio0) interface

Which and why I should choose to use bridge or VLAN? This article[1], do not explain it, nor provide any glue.

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Some notes:
- I'm a gentoo user, so I consider myself a good learner, but haven't yet found any good references to OpenWRT
- My device is TP-Link TL-MR3020[2] by the way it have only one lan/eth0 iface and one wireles/radio0 one.

  [1]: http://wiki.openwrt.org/doc/uci/network/switch
  [2]: http://wiki.openwrt.org/toh/tp-link/tl-mr3020