you have several options here, it all depends on you financial situation.
As mentioned in the answers below, you can shop for your new home and make a contingency on the contract making the closing of your new home dependent on the sale of your current home; you might also consider, renting out your current home
Depending on your credit / income you might qualify for another mortage all together in wich case you could rent the first property and wait for market conditions to be in your favor before deciding to sell.
I would love to discuss further options with you, feel free to call me with any questions you might have
Although you still have a mortgage on your current home, hopefully you have equity. You can list your current home for sale and purchase a larger home contingent on the sale of your existing home. You can pre-qualify for the new mortgage contingent upon the sale. Contact a local realtor who will run the numbers for you.