Not sure what you are using it for, but I have worked 29 years in project engineering and have bought and installed hundreds of tanks.
First things first. Unless you are a plant that builds its own equipment, do not design the tank. It does not pay off especially if used for chemicals or anything regulated.
Second, most tank manufacturers have in house people who can design their tanks in hours and know how they build it. use their expertise. that is why they are in business.
third, draft a design specification. Give them the service volume, the total volume (NEVER fill a tank 100%), operating pressure, design pressure, operating temperature, materials to be stored along with MSDS sheets, all fittings, manholes, instrumentation requirements, piping connections, flow rates, and a general description of how you use the tank. Also determine if the tank must meet government regulations and an industry spec. Is the tank a pressure vessel? Will the tank be inside or outside, heated, insulated or jacketed? What type of foundation will it sit on. Where is it and what is your access to install it? how do you rig the tank to set it and how close can the delivery vehicle get? What kind of rigging fixtures are needed. I always add a crude drawing showing the nozzles and fittings from each end and the side. Show them as degrees from 0 ref. and elevation from bottom of tank. Also remember you need to allow maintenance to service things. If insulated allow space.
Take the time to answer those issues, write up the spec, and send it out for comment in your organization. Then send it out to bid with at least 3 companies. Tell them to deliver at least 4 weeks prior to when you need it. It stinks to coordinate a delivery just in time during plant construction. Go to the supplier when they have basically assembled the tank in case there is an issue. Cheaper to alter the tank there.
Good luck.