> Where should I be aiming? Straight out
>almost at my chest level or up more towards the jaw?
If you want to be authentic, aim the jabs and crosses at nose- or chin-level of your opponent (hitting someone in the chest doesn't do much damage;-) ). For hooks, aim at the side of the chin or side of the temple (the latter could kill somebody if used in real life). For upper cuts, go for under your invisible opponent's chin (the upper cut and hook are both 'knock out' punches).
For side kicks, get the basic movement for the kick by stomping your foot down. Now move that 'stomp' to the side, making sure to chamber (bend the knee) at the beginning and end of the move (which keeps your 'opponent' from grabbing your leg). The knee should face forward. The whole kick is an "in-out-in-down" series of moves.
For roundhouses, bring up your bent leg to the side, so your knee is pointing towards your target. Keep your foot pointed, then snap the lower leg in and out, then put foot down: again, in-out-in-down. If you want to work the hamstrings more, emphasize the chamber after the kick (ie: bringing the foot quickly back to a bent position).
If your knee is pointed forward on kicks, they should look 'straight' from the front. However, you don't really want your leg to be 100% straight, as that can be bad for the knee (especially if you aren't actually kicking something that will absorb the force of the kick). Instead, don't extend to more than about 90-95% of straight, and focus on quickly recoiling your leg.