I just found out my friends are also going to Yosemite the same time I am. They have all backpacked the John Muir Trail but I have never done multi-day backpacking. I need to pick up some gear. Any advice for a first timer like me? What kind of a lightweight pack to get, sleeping bag, mat, tent, etc.?

My first suggestion to anyone who's never gone before and has nothing - rent/borrow your gear. There is an REI in Fresno if you want to pick it up on the way. See if your buddies will share things like stove, water filter, tent, etc. You will be able to rent a sleeping bag and pad, and a bear canister - if you are willing to carry a Garcia the park will rent one for five bucks for the duration of your trip, if you want the clear widemouth Bear Vault you can get one at REI.

Rent or borrow because if you come back from the trip and vow to never go out again, you won't have spent 2,000 bucks on gear. The selection won't be great but you will do just fine especially if you share the load with others.

I suggest bringing lots of snacks to munch on all day. If you buy Mountain House or similar prepackaged meals, split the meals into half in ziplocs - that way when you lose the appetite the first couple of days you won't waste a ton of food. And if you don't lose the appetite, you can just eat it anyway!

Just got back from Yosemite. Hiked the Valley-Tuolumne-Donohue Pass.

You question is very general. You did not specify exactly when you are going but I'll assume it's soon. I don't know much about your history or personal preferences. I don't know what your budget is. I don't know how much comfort you're willing to give up to save weight. But I can give you some general advice if I make some general assumptions:

Be prepared for any kind of weather, be it heat, rain or even snow. Fall is fast approaching. Nights can dip below freezing even at moderate altitudes. The good news is, the skeeters have had their run. I saw very few, at least in the part of the Park I was in.

Needless to say you will need a bear canister unless you're always camping near a food locker. Which means a pack that can handle a bear canister.

Don't try to go ultralight your first time out. A little more weight can be mean a lot more comfort. On the other hand, if your pack weighs 60 pounds you're probably carrying too much. Try to pool equipment such as stoves and water filters with your friends.

Unless money is no object for you, I'll second the idea of renting/borrowing gear. At this point you have little idea of what works or does not work for you. This can be a very expensive learning curve. For instance, do you prefer a tent or a tarp? Each has its advantages but without experience you really don't know.

Have fun and be sure to give us a TR when it's over so other beginners can learn from your experiences.