Don't worry about canning your jam if this is your first try at it. Make a couple of jars that you can taste and enjoy and learn from.
Decide if you want your balance to be more on the strawberry or the rhubarb side (each are surprisingly assertive), and purchase accordingly--don't get more than about two pounds altogether. Wash, hull, trim both; slice the rhubarb in about 1/3" pieces; quarter your strawberries. Weigh what you've got left of your fruit, and put them in a bowl with 1/2 their weight in sugar. Stir them and leave them at room temperature overnight.
In the morning, strain the liquid into a wide-bottom pot--even a straight-sided frying pan will do. Also put a kettle of water on to boil, and when it does, fill your washed jam jars with it. Boil the fruit liquid down until it has reduced a bunch, but try not to scorch it. Add the fruit and cook it until when you scrape your spatula across the bottom, you can see the bottom of the pan for a second or two. Turn off the heat, pour the water out of your jars (giving them a shake to get out every drop), and fill them with your beautiful jam.
If you want to follow Will Owen's advice, put the strawberries and the rhubarb in separate bowls with their sugar, and cook the rhubarb in the reduced liquid first.
i do about 1 qt of strawberries - just less than 2 cups trimmed
1 1/2 lbs rhubarb cooked in water then drained
add these to a stockpot with a pouch of pectin - stir and bring to boil
then stir in 1/2 -1 tbsp vanilla and sugar... i've read recipes that call for about 6 cups, but i tend to use about 5 (3 white, 2 brown - not packed) - bring to a rolling boil for a minute and keep stirring
ladle into sterilized jars within 1/8 in from top. stir to settle out bubbles, then process for 10 min (depending upon altitude)
Truthfully, I always just use the recipe on the pectin package, but reduce the pectin by a tablespoon or so -- rhubarb has a lot of natural pectin, so if I use the full amount it sets up too hard for my preference. It's a very simple, straightforward recipe without any fancy additional flavorings and turns out a nice sweet-tart jam.
ETA: Jam-making is SO simple, I would say probably the simplest of all home preserving projects, and strawberry-rhubarb is one of the most failsafe of all jams! So it's really a fun project, easy and (I think) therapeutic, with a big tasty reward. That jam on a hot biscuit? Divine. :) Enjoy!
I set out to make a compote of rhubarb and strawberry I used to do years ago, and wound up overcooking everything. Nothing but rhubarb (cut into 1" pieces), strawberries, water and sugar, and it's almost jam as it is! In fact you could spread this if you wanted to, though it's still a bit runny. So: if I can do it by accident, any idiot should have no trouble doing it on purpose! Oh - it IS delicious, by the way. And I added some vanilla/fig flavored balsamic vinegar we found last summer, which is killer good, but a bit of regular supermarket balsamic would do nicely.