Talk to a person. After you introduce yourselves to each other and speak with one another, select the option "See you later" to end the conversation. This will increase your Friendship Level each time.
Health Bar
For those who didn't know, when you open up your menu and on you Status, the Strength bar only increases when you eat, but there is an easier way to do it than buying a lot of Turnips and what not.
Go to a Savory Pancakes Store and start ordering the first menu Item, Savory Pancakes. This is a mere $7, but if you don't have around $1500 in your wallet than just wait until June or December for your Salary. Anyway if you keep doing this, you will have a full Strength bar in no time. This helps you in two ways and harms you in one way. You can Zoom Across town in a long jump. You don't see any of the "A bit Hungry" stuff.