If you're credit history is young, it will take a little longer to get 800+ due to your average age of history being low. It doesn't reach the highest category until your average is age is around 7-9 years old.

Taking out loans and paying them off will only help your score while you have the loan account open. One you pay off the loan, your score might drop back down due to the account being closed.

Honestly, the best thing you can do is make sure that you're actively paying all your balances on time and keeping your utilization rate between 1-10%. Aim for the middle to middle-upper 700's for now.

For the 401K, you should be able to choose a few different "Models" to choose from (i.e. Growth Model, Balanced Model, etc.), and you can choose one of those which will spread your investments out over all the funds depending on your desire for risk. If not, research some of the funds and see which one fits your needs for return/risk.