Hey Guys
We are young Family from Australia and looking at staying in Yosemite and Lake Tahoe in July
Can anyone suggest some great accommodation and things to do
Thanks

How long, and what is your budget?

July is the main high season for both areas (the week around July 4th is the biggest holiday period of the entire summer and all schools are out so most families vacation around that time)

It will be VERY hard to get accommodations in Yosemite Valley -- so I would do a 'provisional' booking at Yosemite View Lodge in El Portal and then keep trying for a cancellation in the Valley. Do NOT stay farther out than El Portal. Places that claim to be close to the park are generally a 90 min to 2 hour drive away.

Tahoe will be easier because there is soooooooo much inventory. But the specific dates and your budget can make a huge difference.

Here is the new YARTS (Yosemite Area Rapid Transit System) bus schedule. http://yarts.com/routes-schedules/ The new schedule goes into effect on May 9, 2016.
If you are coming through Merced, janisj is right, stay in El Portal.
If you are coming from the south, you can get the Rt. 41 bus from the Fresno airport (FAT) or the Fresno Amtrak station (FNO).
It is possible to get to Yosemite from San Francisco or Los Angeles without a rental car.

>>It is possible to get to Yosemite from San Francisco or Los Angeles without a rental car.

Yeh we will be hiring a car no dramas with transport
what is the best area to stay in Tahoe or are all places good

We are looking at the dates of July 21st to 24th for Yosemite and 25th to 29th Tahoe

If it was me -- I'd try to book in Yosemite Valley or in El Portal first. Availability will be very limited if at all. Then build the rest of your itinerary around which dates you can get there - if any.

There are maybe areas all around Lake Tahoe some like the action at Stateline/south shore, ome like the lower key north shore, some like the quiet west shore. What is your budget?