All you have—if you don't have experience—is rules.
I found this on a tennis site today and thought maybe you could use it. Remember all "rules" in tennis can be broken, and sometimes should be, but not by beginning players.
Notice: doubles offers more choices, but some of them are bad.
Singles
From the baseline, cross court is an easier and higher percentage target than down the line.
Down the line absolutely CAN be the right shot selection, but you should only target that direction when you have a good reason to, or multiple "good reasons" stack up to reward you for making the more difficult shot.
Remember, if you DO pull the trigger give yourself lots of margin for error. Don't fall for the sucker play of aiming anywhere close to the lines.
When you're at the net, keep the ball in front of you any time you're off balance or in trouble, and only hit cross court when you have the upper hand with a good opportunity to finish the point.
Doubles
In the majority of doubles points you have the choice of aiming towards a closer player or a player further away from you.
Sometimes its really obvious and clean cut, like when you're returning serve. Other times your opponents will be closer to the same distance from the net, but usually its pretty clear who's in a more offensive position and who's in a more defensive one.
Paying close attention to that and choosing your target based on your phase of play is the key to smart, winning doubles.
When you're back at the baseline your primary goal is to avoid the more offensive opponent (the one closer to the net) at all costs. The only exception is when you have a clear opportunity to attack with a groundstroke.
If you're at the net and have an offensive shot (high contact point, ability to hit down) aim TOWARDS the closer player the vast majority of the time. Attacking towards the opponent who's further away will result in resetting the point.
When you're at the net and in a defensive situation (low contact point or stretched out) keep it AWAY from the closer player at all costs.
Finally, if you have a neutral ball (not clearly offensive or defensive) it's your choice, but if you're not confident in hitting something to make the close opponent uncomfortable your default should be going back towards the deeper one.