How to read a paper? Start to finish is not always feasible nor advisable....
If you are starting to search for scholarly articles in an area that you are completely unfamiliar with, make sure to assign some time to get familiar with terminology that is being used. This includes opening links to papers and reading the abstract sections, looking at the keywords usually listed immediately after the abstract section, and ensuring that you are taking the time to look up meanings of words that are new to you, and maybe even making some flash cards or word lists.
After sometime, you will find yourself using some of the new words that you were introduced to as keywords in your search process. It's totally normal to not know all the key words in an area of research. A part of the research process involves skimming through papers to spot these words and to get familiar with them. Do this before you actually start searching for papers that are relevant to your area of interest.
Once you think you have found a paper that interests you, you should still spend a little glancing through its sections to see if it's really a paper that will help you at the moment and to see which sections are relevant for your right now. Remember, you can always revisit a paper later on.
This isn't a complete list of goals but the more focused you are, the easier it becomes to decide which section of a research article you need to pay attention to. However, keep in mind that review papers don't always conform to the classic sections that you are familiar with in a primary research article.
Of course, as you continue your research process you will come across papers that you will read from beginning to the end. So, it important to prioritize what you spend your time on, early in the process.
When you find a paper and read it's title and you are still not sure if it's a paper relevant to your work, definitely skim through the abstract of the paper. When you are starting off with the research process even reading abstracts can seem hard and overwhelming. Don't worry, you will get better with practice. Also, the key isn't to understand everything. The key is to get enough information to gauge whether the paper is worth exploring further for your current project or assignment.
Ideally, after reading an abstract you can answer some questions.
Is this paper relevant to your area of interest? If yes, save it. If no, move on to the next one.
Is this an empirical study or a review paper? Review papers are a good way to catch up on several years of research in an area. They tend to be longer articles but they can give you deep insight into the history of research trends in a field.
If you found that a paper is relevant to your field, you should still take a couple of minutes to skim through it to confirm if it's really relevant to your research process. This will help prevent something that happens very often during the process of research, which is four hours later, after going down several different subtopic routes, you are no where close to feeling confident about setting down on an area of research. While this isn't necessarily a bad thing, it's just important to learn how to manage time constructively during the research process.
Break down the research process with smaller steps and time box these activities based on your reading and comprehension comfort levels.
Most importantly take notes systematically throughout the process. These notes are for you, so they don't need to be beautiful but try to have them stored in a manner that lets you find them easily (think reference manager like Zotero).