Creating your Sim's physical appearance
This is usually the most time-consuming portion at the outset of any game in which you're given free reign over how your character should look, but the process is made slightly easier if you plan for your household to be comprised of people you know, popular characters from an 80's John Carpenter film perhaps, or maybe even staff members from online gaming publications.
The panel in the top left lets you choose the name, sex, age group, gait, the pitch and tone of your Sim's voice and the nature of their relationship to the other Sims in the same household. The avatar you'll be presented with is randomised so to start tweaking its appearance to match the visage of your dreams, just click on whatever part of the body you want to start altering to bring up the next menu.
All of the features have a number of pre-sets to choose from that can be further altered by clicking and dragging on the desired body part after selection to amend the size. The clicking and dragging method also applies to larger body parts in general like legs, arms, etc. In fact, after extensive grabbing, pushing and pulling, it seems that the only area of the body that this doesn't apply to is the crotch, so yanking away at the thing is fruitless.
You can also prepare the outfits for your Sim for 5 difference occasions detailed at the top of the right panel which they'll helpfully change themselves into at the appropriate time. There'll be no giant hotdog outfits at those formal affairs unless you decree it, which is highly recommended.