Unfortunately we do not have on-staff expertise in the pest control area. However, the short answer is that you will need to trap/kill current rodents and then seal all possible entry points to prevent subsequent infestation. You may do this yourself, but may require hiring a professional to remove existing animals in your attic and then identify and seal all possible entry points. Identifying entry points can be the most difficult step as mice and chipmunks can fit through incredibly small openings.

From the internet, below are some resources that may further help find a solution (the superscripts will hyperlink to external websites with more information):

Exclusion (Prevention)
The most effective long-term solution is exclusion—physically preventing the animals from entering.

• Seal Entry Points: You should inspect the exterior of your home for holes where gas lines, cooling lines, electric cables, or vents enter the house and seal them [³].
• Materials: Use durable materials that animals cannot chew through. "Hardware cloth" (welded wire mesh) with 1/4-inch mesh is highly recommended for closing entrances and openings [¹, ⁴, ⁶]. You can also use caulk for smaller gaps [¹, ³].
• Gaps: Ensure you attach wire mesh coverings to rain gutter downspouts or vent openings, as these are common entry points [³].

Removal
If the animals are currently inside, you will need to remove them before permanently sealing the entry points.

• Trapping: Trapping is considered the most practical method for eliminating chipmunks and similar rodents in home situations [³, ⁴]. You can use common rat snap traps or live traps (wire mesh box traps) [³, ⁴].
• Bait: Effective baits include peanut butter, oatmeal, nut meats, pumpkin seeds, or raisins [³, ⁴].
• Placement: Place traps along runways or near the entry points you have identified [³].

Important Considerations

• Juveniles: Be aware that if there are animals in your attic, there may be babies present, particularly in the spring or late summer. If you trap a mother and seal the hole, the young may be left behind to die, which can cause odor problems and attract insects [²].
• Repellents: Most repellents rely on bad smells or tastes. Because the attic space adjoins your living space, use caution with repellents as the odor may drift into the rooms below [⁶].
• Poisons: Poison baits are generally recommended for use outside only, as animals may die inside the wall voids or attic, leading to odor issues [³].

Michigan Resources
Since you are located in Michigan, it is helpful to know that Michigan State University Extension does not have a group that deals solely with residential animal control [⁵]. However, the Michigan Department of Natural Resources (DNR) provides information on dealing with nuisance wildlife and maintains a list of licensed animal control companies if you prefer to hire a professional to handle the removal and exclusion [⁵].

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