Start by picking out things that you are drawn to.
If you are overwhelmed, limit your choices.
If your choices are too many, consider rotating your supplies (I have a now stash which I work from daily and a later stash which I rotate from when I get bored every couple of months with my now stash.)
Arrange the elements on a page.
Think of it like you are solving a puzzle. Spend a few minutes (not an hour) seeing which piece you like where.
Notice how every time you move the pieces you get a new collage. It's not that one way is the right way and the other is wrong. It's that there are many possibilities and opportunities!
Repeat the above on a new page.
Make several pages like this, each time trying to push yourself to do something a little different than you did before.
Make several pages before you go back and look at them again.
Wait a day or more.
Step away from your pieces. Evaluate them in a new light.
Does it need something else in this corner? Could you pull more color from somewhere on the page (Maybe there's a hint of blue in the corner that you could repeat elsewhere on the page?) Maybe there's a circle pattern on a piece of paper in that top left corner that you could draw somewhere else to draw your eye into the piece more?
What are you drawn to?
What do you wish you had done differently?
What color palettes do you want to explore?
Whose work are you drawn to? Can you imitate their work and try to learn from it?
Don't compare your work. Learn from others but do not be too harsh.
All work has value.
Push and keep pushing.
Always ask, "what if?"
Don't be afraid to step outside of your comfort zone.
Some Additional Tips:
Use what you have. Even junk mail can be shredded to make collages. Turn magazine images upside down. Don't feel the need to buy what the latest and greatest toys are, use what's on hand and within reach.
When gluing, burnish things down with a dry rag or paper towel. I've found that not only does it help to wipe off any glue that's been smooshed where it shouldn't be but also it helps to make things really stick to the page.
Use an old magazine or catalog as a glue surface. When you're done, pop it in the recycle bin.
The more you work (especially in journal form) you realize what imagery, patterns, colors, symbols, etc... you are drawn to. Don't beat yourself up over not having a visual vocabulary immediately. It will slowly reveal itself to you with time and work. Be open to it. Look for it in the works of other artists.
What advice would you offer to someone who is creating a collage for the first time?