what mods should I get?
Here are a few well known mods with some info to help you choose. :)
Millenaire - Adds lots of naturally generated structures with NPCs, as well as the ability to trade amongst villages, raise relations with villagers, get cool loot, etc.
TMI - In-game inventory editor with the ability to switch between gamemodes, set the time, and toggle precipitation. Basically a neat GUI for controlling the properties of your save.
Optifine - Adds lots of graphics options for configuring the game to be more playable on certain computers.
Fossils and Archaeology - Adds fossils and relics that can be dug up. Fossils can be cultured into eggs through a few machines, allowing you to have pet dinosaurs.
Better Than Wolves - Adds a lot of new items and blocks that stick to the general theme of the game. Greatly expands gameplay.