Just in case you couldn't figure it out yourself, or if you have nothing better to do.
Step 1: Getting Started
If you haven't already, make an account. On the homepage you will find the sign up link in the top right corner. By clicking it you will come to a page with several options. Option one says Get Started, and allows you to get a FREE account, which may be your best option at this time. If you decide that you would rather go the extra mile, opposite of the free account is the Pro Account Pricing. This shows the different prices per month and the payment types that are accepted. You can chose from $1.45/month, billed annually, $2.95/month, billed quarterly, or the best value of $29.95/2 years, a one time payment. Not sure what the big deal is with a Pro account? Fear not, because at the bottom of the page the site lists all advantages of the Pro account compared to a free one. Now, after making your difficult decision, you will come to a screen asking for your email, username, password, age(can be hidden), gender, and ZIP code.
Step 2: Ideas
Now let's assume you don't already have an idea for your instructable. Don't worry. You can look around first by clicking on the Explore button on the homepage, near the top. This lets you explore many many other projects, in areas such as Food, Living, Play, Technology, ect. Too many projects to look at? Well, you can also use other search options to see editor's picks, popular projects, recent projects, and most viewed. Still not sure what to do? That's why they have contests. Right next to the Explore option is the Contest option. Here, they have various contests for different types of projects, from gaming to school teaching, all with many possible prizes. By now you should have an idea for your own instructable, but if not, just keep searching. I'm sure you'll find one.