Know your goals and your growing environment, and choose a strain based on those characteristics.
Choosing a strain may seem overwhelming at first, but by answering a few simple questions, you will be able to narrow down the choices to strains that suit you best.
Do you plan on growing INDOORS or OUTDOORS?
Do you prefer the characteristics of an INDICA (i.e. short, compact, and high yielding)? Or do you prefer the characteristics of a SATIVA (i.e. tall, high quality bud, and lower yielding)? Or would you prefer a mix between the two (i.e. 50/50 or 60/40 cross)?
What type of high are you looking for? There are two basic types: The first is a "body" or "couch" high that knocks you down and holds you there, which is synonymous with INDICAS. A second type of stone is a more psychedelic and "cerebral" high, often associated with SATIVAS.
How potent do you want it? Different strains have their own unique effects (i.e. mellow, happy, paranoid, stupefying, clear and thought provoking, etc, etc…)
What is your price range? How much are you willing to pay for seeds or clones?
Is yield important to you? There are low, medium and high yielding cannabis plants. Does yield take precedence over quality?
Is odor or smell a concern? Stealth growing requires low smelling strains. Some strains smell so strong they may be a security risk.
Is taste important for you? (Some strains have a unique flavor such as grapefruit, blueberry, etc)
How long do you want to wait to harvest? Some are 6 week cash-croppers, some will take up to 14 weeks to flower. Many strains take 7-8 weeks.
The last thing you should do when you have a few strains narrowed down is ask other growers. Find out what growers have experienced with it, etc.
