I can add some detail later, but just to reassure you - that's a very do-able commute.

You can look at the City of Seattle bike maps to get yourself to the I90 bridge.

Take the I90 bridge to Mercer Island; follow the bike route signs across Mercer Island; take the bike lane across the the bridge to Bellevue. When you get to the end of the bike land on I90, you hook around under I90 and take the bike path across Mercer Slough until you reach 118th Avenue (which is right next to the 405.) You take 118th Avenue North until you are in downtown Bellevue. Then ease your way through downtown Bellevue to your job.

118th Ave is the flattest way I know to get from I90 up to downtown Bellevue. It's a nice wide street w/some traffic but I feel safe on it. There are some prettier ways to get from I90 up to downtown Bellevue but they're hillier.

This all seemed a little complicated the first couple times I did it but it's a very easy route once you know how to get to the I90 bike path; get across Mercer Island, and get through Mercer Slough to 118th.

The prettiest way through Bellevue is along Killarney Way and 98th Avenue, but there are some pretty good climbs on that route, especially going North to South (the climb up 98th Ave is a big one.)

(you can also ask the moderators to move this over to the Pacific Northwest Forum where you can get other Seattle-specific advice if you want it...).

There is a good bike map for Bellevue on the city of Bellevue's website that will show you the detail.