You must take an RMD in the same tax year you turn 70 1/2. Now for 2018 there are 365 days in this year. On day 183 you will turn 70. On day 366 you will turn 70 1/2. Since there's only 365 days in 2018, that means you turn 70 1/2 on Jan 1, 2019. So you should be fine waiting until 2019 if you want. However, I would *HIGHLY* suggest you confirm this with a tax professional that you can hold legally liable and financial culpable for the advice they give you. You don't have that in this public user to user forum.
Here is a link to IRS site that explains this. It specifically notes the case of a July 1st birthday.
Your RMD year is 2019. You get a small extra time period for the first RMD, so you will need to take your first RMD by April 1, 2020. However, it is usually NOT recommended to wait beyond 2019 for your first RMD because you will have your second RMD, for Tax Year 2020, that will be due by December 31,2020. Taking 2 RMDs in the same tax year often bumps you into a higher tax bracket. So the normal recommendation is to take your first RMD in 2019 and your 2nd RMD in 2020.