since everyone is busy jerking each other I'll actually explain it:
Reddit is made up of tons of "subreddits" - these are 'channels' with a specific name like r/pics, r/nature, r/imgoingtohellforthis ( urls will look like www.reddit.com/r/imgoingtohellforthis )
There are default subreddits that your account is already subscribed to. You can change this by clicking the pencil in the top right corner and hitting "edit frontpage subscriptions"
Your frontpage is your selection of subreddits that will all pool together on your "front page" when you log in. You can customize your experience by limiting and including subreddits of your choice. Think of your interests. Into sewing? Theres probably a subreddit out there for it. If you subscribe to it, it will be included in your front page feed. If you can't find the name of the subreddit a quick google search will help you find the exact r/[name_here] .
Most people are fine with the default subreddits. It includes r/pics, r/worldnews, r/funny, r/aww, and other relatively general topics. You can also view "all" to see the top scoring posts from every subreddit on reddit.
What is a score? Everyone has one upvote or downvote per post. The score for a particular post is the upvotes minus the downvotes. The score is displayed between the downvote and upvote marker to the left of the link. It gives a general idea of how popular the post has become. This allows the community as a whole to decide what is interesting or not. This of course has led to much debate (see 'Karmawhoring'). Karma is another word for how many upvotes your post or comment received. Post karma and comment karma are delineated and can be seen next to your account name in the top right, in that order.
hope this helps.