Ask yourself:
1. What type of problem is it? What type of physics?
By "type of problem" I mean e.g. projectile motion, relative motion, balancing forces, rotational motion, energy, etc. To identify the problem, look for the quantities described by phrases like “force”, “velocity with respect to”, and take a look at the picture, if there is one. More complicated problems might combine two or more "basic" types of problems.
Then look at only the equations that pertain to that particular type of problem (equation sheets tend to organize the equations so that this is a bit easier for you, and if you make your own sheet, organize it). For example, in the case of a relative motion problem, that doesn’t ask about forces, energy, and nothing is rotating, you could exclude the latter three groups of equations
2. What is given by the problem? What are we looking for?
Look at the text and figure (if provided) for anything that is given i.e. the direction and magnitude of a force, and initial velocity, etc. Sometimes information is provided without numbers, e.g. “the block starts from rest” -> initial velocity = 0.
Also consider things assumed that might not be written (the most common example is that acceleration in the y-direction is gravity!). I really recommend writing these down on a sheet of paper yourself, not just highlighting all over the page, this helps keep you focused and organized.
Finally, write down what the problem is looking for. For example, the final velocity, v_final = ?
3. What equation can/should I use?
Look at what information you have and what information you need - try to find an equation that includes both of these, without anything extra.
If you can’t find a “perfect” fit, perhaps choose an equation with two unknowns, you might need to write a second equation to solve for one unknown and then eventually figure out the final answer. Likewise, you may need three, or more, but generally start from the/an equation that has (ideally) only the variable you are looking for and lots of variables you already know.
Then, plug in and solve!
- Use units! (using units is more tedious, but is very valuable and allows you to check your answer – for example if you think you solved for time, but your final units are m/s, or 1/s^2, you did something wrong)
- Use negative signs depending on how you define your system (i.e. up is positive, down is negative) – be consistent!
- Break up x- and y-directions from the very beginning (this means that when you write your known variables you should already denote the directions (for example you might know the initial velocity in the y-direction but not the initial velocity in the x-direction). Only combine at the end (if needed!).
- Ask yourself if your answer makes sense at the end – a boat only taking 0.1 seconds to cross a wide river is unrealistic, and likely wrong.