The official support page for Pokémon Go has a great wealth of information to guide new trainers on to the proper course.
Start Off With a Pikachu
At the start of the game, Professor Willow will guide you through catching your first Pokémon – Bulbasaur, Charmander or Squirtle. There is a way to start with Pikachu if you wish to.
After playing for some time, you would have noticed that PokéStop and gyms are not equally distributed. Same goes for Pokémon spawning rate and density. Highly populated areas such as cities and malls will have more Pokémon than rural and residential areas.
Never Power Up
As tempting as it is to power up and make your Pokémon stronger, don’t do it yet. Pokémon are replaced every few levels as you will find stronger Pokémon as you level up. The stardust and candy could be used more efficiently. Candy is a valuable resource as a low level trainer can use it for power leveling to gain few levels in less than an hour.