Talk about the story of my life. Self-taught in Access, VBA and JetSQL, then graduated to SQL Server. It's a daunting task, but not impossible. Just eat the elephant one bite at a time.

As far as SQL Server goes, get a full-blown copy if budget allows and park it on it's own server. You'll save yourself a lot of headache in the long run.

SQL Server Express is for the most part a viable option if cost is a consideration and scale isn't too big, but read up on the limitations. Most probably aren't a concern, but you'll definitely want to find a third party tool (or roll your own) for scheduled jobs since SSE lacks the Job Agent. You'll eventually want to automate as much as you can (backups!!).

JetSQL will familiarize you with the basics of SQL syntax, just be aware it has it's own idiosyncracies and T-SQL has no knowledge of the Access Object Model unlike JetSQL.

Overall, treat it as a tiered solution and make your data as independent as possible from your Access front-end. If the front-end has pretty much only Forms, Reports and just enough VBA to make it work, you're doing well. This also gets you to the point where you can create an executable in VB or C Sharp to replace the Access portion altogether, to say nothing of being able to leverage your data without relying on Access.

In any event, good luck.